Add an ASCII (single-byte) fast path in `String::push`

`String::push(&mut self, ch: char)` currently has a single code path that calls `Char::encode_utf8`. This adds a fast path for ASCII `char`s, which are represented as a single byte in UTF-8.

Benchmarks of stage1 libcollections at the intermediate commit 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).

```
bench_push_char_one_byte                  59552 ns/iter (+/- 2132) = 167 MB/s
bench_push_char_one_byte_with_fast_path    6563 ns/iter (+/- 658) = 1523 MB/s
bench_push_char_two_bytes                 71520 ns/iter (+/- 3541) = 279 MB/s
bench_push_char_two_bytes_with_slow_path  71452 ns/iter (+/- 4202) = 279 MB/s
bench_push_str_one_byte                   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 just as well as `s.push('x')`.)