Reduce text size for fail invocations

A few refactorings to decrease text size and increase data size. I'm not sure about this tradeoff. Various stats below. cc @pcwalton

This reduces the code needed to pass arguments for `fail!()`, `fail!("{}", ...)`, and to a lesser extent `fail!("...")`. Still more work to be done on compiler-generated failures and the `fail!("...")` case.

do_fail_empty:

```
#[inline(never)]
fn do_fail_empty() {
    fail!()
}
```

do_fail_empty before:

```
	leaq	8(%rsp), %rdi
	movabsq	$13, %rsi
	leaq	"str\"str\"(1494)"(%rip), %rax
	movq	%rax, 8(%rsp)
	movq	$19, 16(%rsp)
	callq	_ZN6unwind31begin_unwind_no_time_to_explain20h57030457935ab6111SdE@PLT
```

do_fail_empty after:

```
	leaq	_ZN13do_fail_empty9file_line20h339df6a0541e837eIaaE(%rip), %rdi
	callq	_ZN6unwind31begin_unwind_no_time_to_explain20h33184cfdcce4dfd8QTdE@PLT
```

do_fail_fmt:

```
#[inline(never)]
fn do_fail_fmt() {
    fail!("guh{}", "faw")
}
```

do_fail_fmt before:

```
        ... (snip lots of fmt stuff)
	callq	_ZN3fmt22Arguments$LT$$x27a$GT$3new20he09b3a3f473879c41paE
	leaq	144(%rsp), %rsi
	movabsq	$23, %rdx
	leaq	"str\"str\"(1494)"(%rip), %rax
	leaq	32(%rsp), %rcx
	movq	%rcx, 160(%rsp)
	movq	160(%rsp), %rdi
	movq	%rax, 144(%rsp)
	movq	$19, 152(%rsp)
	callq	_ZN6unwind16begin_unwind_fmt20h3ebeb42f4d189b2buQdE@PLT
```

do_fail_fmt after:

```
        ... (snip lots of fmt stuff)
	callq	_ZN3fmt22Arguments$LT$$x27a$GT$3new20h42e5bb8d1711ee61OqaE
	leaq	_ZN11do_fail_fmt7run_fmt9file_line20h339df6a0541e837eFbaE(%rip), %rsi
	leaq	32(%rsp), %rax
	movq	%rax, 144(%rsp)
	movq	144(%rsp), %rdi
	callq	_ZN6unwind16begin_unwind_fmt20hfdcadc14d188656biRdE@PLT
```

File size increases.

file size before:

```
-rw-rw-r-- 1 brian brian 100501740 Jul 24 23:28 /home/brian/dev/rust2/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc-4e7c5e5c.rlib
-rwxrwxr-x 1 brian brian  21201780 Jul 24 23:27 /home/brian/dev/rust2/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc-4e7c5e5c.so
```

file size after:

```
-rw-rw-r-- 1 brian brian 101542484 Jul 25 00:34 x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc-4e7c5e5c.rlib
-rwxrwxr-x 1 brian brian  21348862 Jul 25 00:34 x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc-4e7c5e5c.so
```

Text size decreases by 52486 while data size increases by 143686.

section size before:

```
   text    data     bss     dec     hex filename
12712262        5924997     368 18637627        11c633b x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc-4e7c5e5c.so
```

section size after:

```
   text    data     bss     dec     hex filename
12659776        6068683     368 18728827        11dc77b /home/brian/dev/rust/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc-4e7c5e5c.so
```

I don't know if anything can be learned from these benchmarks. Looks like a wash.

std bench before:

```
test collections::hashmap::bench::find_existing             ... bench:     43452 ns/iter (+/- 2423)
test collections::hashmap::bench::find_nonexisting          ... bench:     42416 ns/iter (+/- 3996)
test collections::hashmap::bench::find_pop_insert           ... bench:       214 ns/iter (+/- 11)
test collections::hashmap::bench::hashmap_as_queue          ... bench:       123 ns/iter (+/- 6)
test collections::hashmap::bench::insert                    ... bench:       153 ns/iter (+/- 14)
test collections::hashmap::bench::new_drop                  ... bench:       547 ns/iter (+/- 259)
test collections::hashmap::bench::new_insert_drop           ... bench:       682 ns/iter (+/- 366)
test io::buffered::test::bench_buffered_reader              ... bench:      1046 ns/iter (+/- 86)
test io::buffered::test::bench_buffered_stream              ... bench:      2156 ns/iter (+/- 801)
test io::buffered::test::bench_buffered_writer              ... bench:      1057 ns/iter (+/- 75)
test io::extensions::bench::u64_from_be_bytes_4_aligned     ... bench:        80 ns/iter (+/- 5)
test io::extensions::bench::u64_from_be_bytes_4_unaligned   ... bench:        81 ns/iter (+/- 6)
test io::extensions::bench::u64_from_be_bytes_7_aligned     ... bench:        80 ns/iter (+/- 4)
test io::extensions::bench::u64_from_be_bytes_7_unaligned   ... bench:        69 ns/iter (+/- 4)
test io::extensions::bench::u64_from_be_bytes_8_aligned     ... bench:        69 ns/iter (+/- 3)
test io::extensions::bench::u64_from_be_bytes_8_unaligned   ... bench:        81 ns/iter (+/- 4)
test io::mem::test::bench_buf_reader                        ... bench:       628 ns/iter (+/- 18)
test io::mem::test::bench_buf_writer                        ... bench:       478 ns/iter (+/- 19)
test io::mem::test::bench_mem_reader                        ... bench:       712 ns/iter (+/- 44)
test io::mem::test::bench_mem_writer_001_0000               ... bench:        31 ns/iter (+/- 1)
test io::mem::test::bench_mem_writer_001_0010               ... bench:        51 ns/iter (+/- 3)
test io::mem::test::bench_mem_writer_001_0100               ... bench:       121 ns/iter (+/- 8)
test io::mem::test::bench_mem_writer_001_1000               ... bench:       774 ns/iter (+/- 47)
test io::mem::test::bench_mem_writer_100_0000               ... bench:       756 ns/iter (+/- 50)
test io::mem::test::bench_mem_writer_100_0010               ... bench:      2726 ns/iter (+/- 198)
test io::mem::test::bench_mem_writer_100_0100               ... bench:      8961 ns/iter (+/- 712)
test io::mem::test::bench_mem_writer_100_1000               ... bench:    105673 ns/iter (+/- 24711)
test num::bench::bench_pow_function                         ... bench:      5849 ns/iter (+/- 371)
test num::strconv::bench::f64::float_to_string              ... bench:       662 ns/iter (+/- 202)
test num::strconv::bench::int::to_str_base_36               ... bench:       424 ns/iter (+/- 7)
test num::strconv::bench::int::to_str_bin                   ... bench:      1227 ns/iter (+/- 80)
test num::strconv::bench::int::to_str_dec                   ... bench:       466 ns/iter (+/- 13)
test num::strconv::bench::int::to_str_hex                   ... bench:       498 ns/iter (+/- 22)
test num::strconv::bench::int::to_str_oct                   ... bench:       502 ns/iter (+/- 229)
test num::strconv::bench::uint::to_str_base_36              ... bench:       375 ns/iter (+/- 7)
test num::strconv::bench::uint::to_str_bin                  ... bench:      1011 ns/iter (+/- 590)
test num::strconv::bench::uint::to_str_dec                  ... bench:       407 ns/iter (+/- 17)
test num::strconv::bench::uint::to_str_hex                  ... bench:       442 ns/iter (+/- 7)
test num::strconv::bench::uint::to_str_oct                  ... bench:       433 ns/iter (+/- 46)
test path::posix::bench::ends_with_path_home_dir            ... bench:       167 ns/iter (+/- 10)
test path::posix::bench::ends_with_path_missmatch_jome_home ... bench:       148 ns/iter (+/- 6)
test path::posix::bench::is_ancestor_of_path_with_10_dirs   ... bench:       221 ns/iter (+/- 31)
test path::posix::bench::join_abs_path_home_dir             ... bench:       144 ns/iter (+/- 23)
test path::posix::bench::join_home_dir                      ... bench:       196 ns/iter (+/- 9)
test path::posix::bench::join_many_abs_path_home_dir        ... bench:       143 ns/iter (+/- 6)
test path::posix::bench::join_many_home_dir                 ... bench:       195 ns/iter (+/- 8)
test path::posix::bench::path_relative_from_backward        ... bench:       248 ns/iter (+/- 10)
test path::posix::bench::path_relative_from_forward         ... bench:       241 ns/iter (+/- 13)
test path::posix::bench::path_relative_from_same_level      ... bench:       296 ns/iter (+/- 11)
test path::posix::bench::push_abs_path_home_dir             ... bench:       104 ns/iter (+/- 7)
test path::posix::bench::push_home_dir                      ... bench:     27311 ns/iter (+/- 2727)
test path::posix::bench::push_many_abs_path_home_dir        ... bench:       109 ns/iter (+/- 5)
test path::posix::bench::push_many_home_dir                 ... bench:     23263 ns/iter (+/- 1726)
test rand::bench::rand_isaac                                ... bench:       884 ns/iter (+/- 31) = 904 MB/s
test rand::bench::rand_isaac64                              ... bench:       440 ns/iter (+/- 126) = 1818 MB/s
test rand::bench::rand_shuffle_100                          ... bench:      2518 ns/iter (+/- 1371)
test rand::bench::rand_std                                  ... bench:       429 ns/iter (+/- 17) = 1864 MB/s
test rand::bench::rand_xorshift                             ... bench:         0 ns/iter (+/- 0) = 800000 MB/s
```

std bench after:

```
test collections::hashmap::bench::find_existing             ... bench:     43635 ns/iter (+/- 4508)
test collections::hashmap::bench::find_nonexisting          ... bench:     42323 ns/iter (+/- 1753)
test collections::hashmap::bench::find_pop_insert           ... bench:       216 ns/iter (+/- 11)
test collections::hashmap::bench::hashmap_as_queue          ... bench:       125 ns/iter (+/- 8)
test collections::hashmap::bench::insert                    ... bench:       153 ns/iter (+/- 63)
test collections::hashmap::bench::new_drop                  ... bench:       517 ns/iter (+/- 282)
test collections::hashmap::bench::new_insert_drop           ... bench:       734 ns/iter (+/- 264)
test io::buffered::test::bench_buffered_reader              ... bench:      1063 ns/iter (+/- 206)
test io::buffered::test::bench_buffered_stream              ... bench:      2321 ns/iter (+/- 2302)
test io::buffered::test::bench_buffered_writer              ... bench:      1060 ns/iter (+/- 24)
test io::extensions::bench::u64_from_be_bytes_4_aligned     ... bench:        69 ns/iter (+/- 2)
test io::extensions::bench::u64_from_be_bytes_4_unaligned   ... bench:        81 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_7_aligned     ... bench:        70 ns/iter (+/- 5)
test io::extensions::bench::u64_from_be_bytes_7_unaligned   ... bench:        69 ns/iter (+/- 5)
test io::extensions::bench::u64_from_be_bytes_8_aligned     ... bench:        80 ns/iter (+/- 6)
test io::extensions::bench::u64_from_be_bytes_8_unaligned   ... bench:        81 ns/iter (+/- 5)
test io::mem::test::bench_buf_reader                        ... bench:       663 ns/iter (+/- 44)
test io::mem::test::bench_buf_writer                        ... bench:       489 ns/iter (+/- 17)
test io::mem::test::bench_mem_reader                        ... bench:       700 ns/iter (+/- 23)
test io::mem::test::bench_mem_writer_001_0000               ... bench:        31 ns/iter (+/- 3)
test io::mem::test::bench_mem_writer_001_0010               ... bench:        49 ns/iter (+/- 5)
test io::mem::test::bench_mem_writer_001_0100               ... bench:       112 ns/iter (+/- 6)
test io::mem::test::bench_mem_writer_001_1000               ... bench:       765 ns/iter (+/- 59)
test io::mem::test::bench_mem_writer_100_0000               ... bench:       727 ns/iter (+/- 54)
test io::mem::test::bench_mem_writer_100_0010               ... bench:      2586 ns/iter (+/- 215)
test io::mem::test::bench_mem_writer_100_0100               ... bench:      8846 ns/iter (+/- 439)
test io::mem::test::bench_mem_writer_100_1000               ... bench:    105747 ns/iter (+/- 17443)
test num::bench::bench_pow_function                         ... bench:      5844 ns/iter (+/- 421)
test num::strconv::bench::f64::float_to_string              ... bench:       669 ns/iter (+/- 571)
test num::strconv::bench::int::to_str_base_36               ... bench:       417 ns/iter (+/- 24)
test num::strconv::bench::int::to_str_bin                   ... bench:      1216 ns/iter (+/- 36)
test num::strconv::bench::int::to_str_dec                   ... bench:       466 ns/iter (+/- 24)
test num::strconv::bench::int::to_str_hex                   ... bench:       492 ns/iter (+/- 8)
test num::strconv::bench::int::to_str_oct                   ... bench:       496 ns/iter (+/- 295)
test num::strconv::bench::uint::to_str_base_36              ... bench:       366 ns/iter (+/- 8)
test num::strconv::bench::uint::to_str_bin                  ... bench:      1005 ns/iter (+/- 69)
test num::strconv::bench::uint::to_str_dec                  ... bench:       396 ns/iter (+/- 20)
test num::strconv::bench::uint::to_str_hex                  ... bench:       435 ns/iter (+/- 4)
test num::strconv::bench::uint::to_str_oct                  ... bench:       436 ns/iter (+/- 451)
test path::posix::bench::ends_with_path_home_dir            ... bench:       171 ns/iter (+/- 6)
test path::posix::bench::ends_with_path_missmatch_jome_home ... bench:       152 ns/iter (+/- 6)
test path::posix::bench::is_ancestor_of_path_with_10_dirs   ... bench:       215 ns/iter (+/- 8)
test path::posix::bench::join_abs_path_home_dir             ... bench:       143 ns/iter (+/- 6)
test path::posix::bench::join_home_dir                      ... bench:       192 ns/iter (+/- 29)
test path::posix::bench::join_many_abs_path_home_dir        ... bench:       144 ns/iter (+/- 9)
test path::posix::bench::join_many_home_dir                 ... bench:       194 ns/iter (+/- 19)
test path::posix::bench::path_relative_from_backward        ... bench:       254 ns/iter (+/- 15)
test path::posix::bench::path_relative_from_forward         ... bench:       244 ns/iter (+/- 17)
test path::posix::bench::path_relative_from_same_level      ... bench:       293 ns/iter (+/- 27)
test path::posix::bench::push_abs_path_home_dir             ... bench:       108 ns/iter (+/- 5)
test path::posix::bench::push_home_dir                      ... bench:     32292 ns/iter (+/- 4361)
test path::posix::bench::push_many_abs_path_home_dir        ... bench:       108 ns/iter (+/- 6)
test path::posix::bench::push_many_home_dir                 ... bench:     20305 ns/iter (+/- 1331)
test rand::bench::rand_isaac                                ... bench:       888 ns/iter (+/- 35) = 900 MB/s
test rand::bench::rand_isaac64                              ... bench:       439 ns/iter (+/- 17) = 1822 MB/s
test rand::bench::rand_shuffle_100                          ... bench:      2582 ns/iter (+/- 1001)
test rand::bench::rand_std                                  ... bench:       431 ns/iter (+/- 93) = 1856 MB/s
test rand::bench::rand_xorshift                             ... bench:         0 ns/iter (+/- 0) = 800000 MB/s
```

This seems a little odd to have a static with the two together, but then it's immediately destructed? Is that because changing `begin_unwind` would involve changing the ast builder?