std and rustc: Convert users of c_str to use .with_c_str
std: removed some warnings in tests.
std: implement Container for CString
std: Add benchmarks to c_str
std: Micro-optimize vec.with_c_str for short vectors

This now makes it unsafe to save the pointer returned by .with_c_str
as that pointer now may be pointing at a stack allocated array.

I arbitrarily chose 32 bytes as the length of the stack vector, and
so it might not be the most optimal size.

before:

test c_str::bench::bench_with_c_str_long ... bench: 539 ns/iter (+/- 91)
test c_str::bench::bench_with_c_str_medium ... bench: 97 ns/iter (+/- 2)
test c_str::bench::bench_with_c_str_short ... bench: 70 ns/iter (+/- 5)

after:

test c_str::bench::bench_with_c_str_long ... bench: 542 ns/iter (+/- 13)
test c_str::bench::bench_with_c_str_medium ... bench: 53 ns/iter (+/- 6)
test c_str::bench::bench_with_c_str_short ... bench: 19 ns/iter (+/- 0)
std: Remove an unnecessary comment from c_str

The documentation for `.with_c_str()` already says that the pointer
will be deallocated before returning from the function.
std: Up vec.with_c_str's stack buffer to 128

This matches @graydon's recommendation.
std: add micro optimization to vec.with_c_str_unchecked

before:

test c_str::bench::bench_with_c_str_unchecked_long ... bench: 361 ns/iter (+/- 9)
test c_str::bench::bench_with_c_str_unchecked_medium ... bench: 75 ns/iter (+/- 2)
test c_str::bench::bench_with_c_str_unchecked_short ... bench: 60 ns/iter (+/- 9)

after:

test c_str::bench::bench_with_c_str_unchecked_long ... bench: 362 ns/iter (+/-
test c_str::bench::bench_with_c_str_unchecked_medium ... bench: 30 ns/iter (+/- 7)
test c_str::bench::bench_with_c_str_unchecked_short ... bench: 12 ns/iter (+/- 4)
std: simplify vec.with_c_str

This also fixes a bug in `vec.with_c_str_unchecked` where we
were not calling `.to_c_str_unchecked()` for long strings.
auto merge of #9352 : erickt/rust/master, r=huonw

One of the downsides with `c_str` is that it always forces an allocation, and so this could add unnecessary overhead to various calls. This PR implements one of the suggestions @graydon made in #8296 for `vec.with_c_str` in that for a short string can use a small stack array instead of a malloced array for our temporary c string. This ends up being twice as fast for small strings.

There are two things to consider before landing this commit though. First off, I arbitrarily picked the stack array as 32 bytes, and I'm not sure if this a reasonable amount or not. Second, there is a risk that someone can keep a reference to the interior stack pointer, which could cause mayhem if someone were to dereference the pointer. Since we also can easily grab and return interior pointers to vecs though, I don't think this is that much of an issue.