Add String::push_with_ascii_fast_path, bench it against String::push

`String::push(&mut self, ch: char)` currently has a single code path
that calls `Char::encode_utf8`.
Perhaps it could be faster for ASCII `char`s, which are represented as
a single byte in UTF-8.

This commit leaves the method unchanged,
adds a copy of it with the fast path,
and adds benchmarks to compare them.

Results show that the fast path very significantly improves the performance
of repeatedly pushing an ASCII `char`,
but does not significantly affect the performance for a non-ASCII `char`
(where the fast path is not taken).

```
test string::tests::bench_push_char_one_byte                 ... bench:     59552 ns/iter (+/- 2132) = 167 MB/s
test string::tests::bench_push_char_one_byte_with_fast_path  ... bench:      6563 ns/iter (+/- 658) = 1523 MB/s
test string::tests::bench_push_char_two_bytes                ... bench:     71520 ns/iter (+/- 3541) = 279 MB/s
test string::tests::bench_push_char_two_bytes_with_slow_path ... bench:     71452 ns/iter (+/- 4202) = 279 MB/s
test string::tests::bench_push_str                           ... bench:        24 ns/iter (+/- 2)
test string::tests::bench_push_str_one_byte                  ... bench:     38910 ns/iter (+/- 2477) = 257 MB/s
```

A benchmark of pushing a one-byte-long `&str` is added for comparison,
but its performance [has varied a lot lately](
https://github.com/rust-lang/rust/pull/19640#issuecomment-67741561).
(When the input is fixed, `s.push_str("x")` could be used
instead of `s.push('x')`.)
Merge String::push_with_ascii_fast_path into String::push.