Reduce fmt's rodata usage by unzipping String and Argument arrays

Based on an observation that strings and arguments are always interleaved, thanks to #15832. Additionally optimize invocations where formatting parameters are unspecified for all arguments, e.g. `"{} {:?} {:x}"`, by emptying the `__STATIC_FMTARGS` array. Next, `Arguments::new` replaces an empty slice with `None` so that passing empty `__STATIC_FMTARGS` generates slightly less machine code when `Arguments::new` is inlined. Furthermore, formatting itself treats these cases separately without making redundant copies of formatting parameters.

All in all, this adds a single mov instruction per `write!` in most cases. That's why code size has increased.

## Example

Expansion of old `println!("x is: {} foobar", 5);`:

```rust
fn main() {
    let x = 5i;
    match (&x,) {
        (__arg0,) => {
            static __STATIC_FMTSTR: [Piece, ..3] = [
                String("x is: "),
                Argument(Argument {
                    position: ArgumentNext,
                    format: FormatSpec {
                        fill: ' ',
                        align: AlignUnknown,
                        flags: 0u,
                        precision: CountImplied,
                        width: CountImplied,
                    },
                },
                String(" foobar"),
            ];
            let __args_vec = &[argument(secret_string, __arg0)];
            let __args = unsafe { Arguments::new(__STATIC_FMTSTR, __args_vec) };

            println_args(&__args)
        }
    };
}
```

With this PR it becomes:

```rust
fn main() {
    let x = 5i;
    match (&x,) {
        (__arg0,) => {
            static __STATIC_FMTSTR: [&'static str, ..2u] = [
                "x is: ",
                " foobar"
            ];
            static __STATIC_FMTARGS: [rt::Argument<'static>, ..0u] = []; // optimized out
            let __args_vec = &[argument(secret_show, __arg0)];
            let __args = unsafe {
                Arguments::new(__STATIC_FMTSTR, __STATIC_FMTARGS, __args_vec)
            };
            println_args(&__args)
        }
    };
}
```
## Wins
```
before
   text    data     bss     dec   filename
  15017   43168       8     58193 x86_64-unknown-linux-gnu/stage3/lib/libarena-4e7c5e5c.so
  66210   71260       4    137474 x86_64-unknown-linux-gnu/stage3/lib/libdebug-4e7c5e5c.so
  31659    3754       4     35417 x86_64-unknown-linux-gnu/stage3/lib/libflate-4e7c5e5c.so
  20138   45502       8     65648 x86_64-unknown-linux-gnu/stage3/lib/libfmt_macros-4e7c5e5c.so
  76981   83547       4    160532 x86_64-unknown-linux-gnu/stage3/lib/libgetopts-4e7c5e5c.so
  10269   44437       8     54714 x86_64-unknown-linux-gnu/stage3/lib/libgraphviz-4e7c5e5c.so
  33546   40371     200     74117 x86_64-unknown-linux-gnu/stage3/lib/liblog-4e7c5e5c.so
 341427  166384      24    507835 x86_64-unknown-linux-gnu/stage3/lib/libnative-4e7c5e5c.so
  47783  207122       4    254909 x86_64-unknown-linux-gnu/stage3/lib/librbml-4e7c5e5c.so
13324534 6303187     368 19628089 x86_64-unknown-linux-gnu/stage3/lib/librustc-4e7c5e5c.so
 236885  111322       4    348211 x86_64-unknown-linux-gnu/stage3/lib/librustc_back-4e7c5e5c.so
20742937 1035619   70464 21849020 x86_64-unknown-linux-gnu/stage3/lib/librustc_llvm-4e7c5e5c.so
1182102  508505    5280   1695887 x86_64-unknown-linux-gnu/stage3/lib/librustrt-4e7c5e5c.so
 242924  690433       4    933361 x86_64-unknown-linux-gnu/stage3/lib/libserialize-4e7c5e5c.so
 834141 1656905     344   2491390 x86_64-unknown-linux-gnu/stage3/lib/libstd-4e7c5e5c.so
 151137  671324       4    822465 x86_64-unknown-linux-gnu/stage3/lib/libsync-4e7c5e5c.so
3322627 3880814       8   7203449 x86_64-unknown-linux-gnu/stage3/lib/libsyntax-4e7c5e5c.so
 226532  107796       4    334332 x86_64-unknown-linux-gnu/stage3/lib/libterm-4e7c5e5c.so
  46988   66785       4    113777 x86_64-unknown-linux-gnu/stage3/lib/libtime-4e7c5e5c.so

after
   text    data     bss     dec      filename
  15472   43201       4      58677   x86_64-unknown-linux-gnu/stage3/lib/libarena-4e7c5e5c.so
  66250   68591       8     134849   x86_64-unknown-linux-gnu/stage3/lib/libdebug-4e7c5e5c.so
  31659    3752       8      35419   x86_64-unknown-linux-gnu/stage3/lib/libflate-4e7c5e5c.so
  20322   43650       4      63976   x86_64-unknown-linux-gnu/stage3/lib/libfmt_macros-4e7c5e5c.so
  78261   78171       4     156436   x86_64-unknown-linux-gnu/stage3/lib/libgetopts-4e7c5e5c.so
  10221   44019       4      54244   x86_64-unknown-linux-gnu/stage3/lib/libgraphviz-4e7c5e5c.so
  34034   33703     200      67937   x86_64-unknown-linux-gnu/stage3/lib/liblog-4e7c5e5c.so
 345751  153198      24     498973   x86_64-unknown-linux-gnu/stage3/lib/libnative-4e7c5e5c.so
  48403  196039       8     244450   x86_64-unknown-linux-gnu/stage3/lib/librbml-4e7c5e5c.so
13491702  5253560     368 18745630   x86_64-unknown-linux-gnu/stage3/lib/librustc-4e7c5e5c.so
 239161   99363       4     338528   x86_64-unknown-linux-gnu/stage3/lib/librustc_back-4e7c5e5c.so
20742865  1035050   70464 21848379   x86_64-unknown-linux-gnu/stage3/lib/librustc_llvm-4e7c5e5c.so
1186130  480401    5280    1671811   x86_64-unknown-linux-gnu/stage3/lib/librustrt-4e7c5e5c.so
 245928  668157       8     914093   x86_64-unknown-linux-gnu/stage3/lib/libserialize-4e7c5e5c.so
 838093 1597231     344    2435668   x86_64-unknown-linux-gnu/stage3/lib/libstd-4e7c5e5c.so
 152177  653764       4     805945   x86_64-unknown-linux-gnu/stage3/lib/libsync-4e7c5e5c.so
3349091 3563279       8    6912378   x86_64-unknown-linux-gnu/stage3/lib/libsyntax-4e7c5e5c.so
 229488  101380       4     330872   x86_64-unknown-linux-gnu/stage3/lib/libterm-4e7c5e5c.so
  48156   54777       4     102937   x86_64-unknown-linux-gnu/stage3/lib/libtime-4e7c5e5c.so
```

## Difference (negative is better)
```
   text      data    bss     dec   filename
    455        33     -4       484 libarena-4e7c5e5c.so
     40     -2669      4     -2625 libdebug-4e7c5e5c.so
      0        -2      4         2 libflate-4e7c5e5c.so
    184     -1852     -4     -1672 libfmt_macros-4e7c5e5c.so
   1280     -5376      0     -4096 libgetopts-4e7c5e5c.so
    -48      -418     -4      -470 libgraphviz-4e7c5e5c.so
    488     -6668      0     -6180 liblog-4e7c5e5c.so
   4324    -13186      0     -8862 libnative-4e7c5e5c.so
    620    -11083      4    -10459 librbml-4e7c5e5c.so
  167168 -1049627      0   -882459 librustc-4e7c5e5c.so
   2276    -11959      0     -9683 librustc_back-4e7c5e5c.so
    -72      -569      0      -641 librustc_llvm-4e7c5e5c.so
   4028    -28104      0    -24076 librustrt-4e7c5e5c.so
   3004    -22276      4    -19268 libserialize-4e7c5e5c.so
   3952    -59674      0    -55722 libstd-4e7c5e5c.so
   1040    -17560      0    -16520 libsync-4e7c5e5c.so
  26464 -  317535      0   -291071 libsyntax-4e7c5e5c.so
   2956     -6416      0     -3460 libterm-4e7c5e5c.so
   1168    -12008      0    -10840 libtime-4e7c5e5c.so

    -1356 liballoc-4e7c5e5c.rlib
     1540 libarena-4e7c5e5c.rlib
      160 libarena-4e7c5e5c.so
   -74062 libcollections-4e7c5e5c.rlib
        0 libcompiler-rt.a
  -448244 libcore-4e7c5e5c.rlib
   -58910 libdebug-4e7c5e5c.rlib
      294 libdebug-4e7c5e5c.so
      120 libflate-4e7c5e5c.rlib
       -8 libflate-4e7c5e5c.so
   -58534 libgetopts-4e7c5e5c.rlib
    -4443 libgetopts-4e7c5e5c.so
    -1992 libglob-4e7c5e5c.rlib
     -767 libglob-4e7c5e5c.so
    -4686 libgraphviz-4e7c5e5c.rlib
     -384 libgraphviz-4e7c5e5c.so
   -39866 libgreen-4e7c5e5c.rlib
    -4547 libgreen-4e7c5e5c.so
        0 liblibc-4e7c5e5c.rlib
   -66714 liblog-4e7c5e5c.rlib
    -9066 liblog-4e7c5e5c.so
        0 libmorestack.a
  -140108 libnative-4e7c5e5c.rlib
   -10987 libnative-4e7c5e5c.so
   -56826 libnum-4e7c5e5c.rlib
    -2697 libnum-4e7c5e5c.so
   -31722 librand-4e7c5e5c.rlib
  -146418 librbml-4e7c5e5c.rlib
   -10939 librbml-4e7c5e5c.so
  -261958 libregex-4e7c5e5c.rlib
   -22534 libregex-4e7c5e5c.so
        0 librlibc-4e7c5e5c.rlib
  -187366 librustrt-4e7c5e5c.rlib
   -23967 librustrt-4e7c5e5c.so
  -291182 librustuv-4e7c5e5c.rlib
   -19400 librustuv-4e7c5e5c.so
    -4722 libsemver-4e7c5e5c.rlib
     2388 libsemver-4e7c5e5c.so
  -284034 libserialize-4e7c5e5c.rlib
   -19235 libserialize-4e7c5e5c.so
  -655282 libstd-4e7c5e5c.rlib
   -56009 libstd-4e7c5e5c.so
  -328036 libsync-4e7c5e5c.rlib
   -16513 libsync-4e7c5e5c.so
   -74328 libterm-4e7c5e5c.rlib
    -3352 libterm-4e7c5e5c.so
  -198478 libtest-4e7c5e5c.rlib
   -16490 libtest-4e7c5e5c.so
  -148164 libtime-4e7c5e5c.rlib
   -10166 libtime-4e7c5e5c.so
    -1050 libunicode-4e7c5e5c.rlib
   -38478 liburl-4e7c5e5c.rlib
    -5015 liburl-4e7c5e5c.so
   -35978 libuuid-4e7c5e5c.rlib
    -3100 libuuid-4e7c5e5c.so
 -3873611 total
```
Can you add doc comments for these fields? (e.g. explaining what 'simple' means here.)