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)