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).

Output of `make check-stage1-collections NO_REBUILD=1 PLEASE_BENCH=1 TESTNAME=string::tests::bench_push`

```
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')`.)

src/libcollections/string.rs

@@ -528,6 +528,29 @@ impl String {
         }
     }
 
+    #[inline]
+    fn push_with_ascii_fast_path(&mut self, ch: char) {
+        if (ch as u32) < 0x80 {
+            self.vec.push(ch as u8);
+            return;
+        }
+
+        let cur_len = self.len();
+        // This may use up to 4 bytes.
+        self.vec.reserve(4);
+
+        unsafe {
+            // Attempt to not use an intermediate buffer by just pushing bytes
+            // directly onto this string.
+            let slice = RawSlice {
+                data: self.vec.as_ptr().offset(cur_len as int),
+                len: 4,
+            };
+            let used = ch.encode_utf8(mem::transmute(slice)).unwrap_or(0);
+            self.vec.set_len(cur_len + used);
+        }
+    }
+
     /// Works with the underlying buffer as a byte slice.
     ///
     /// # Examples
@@ -1408,6 +1431,63 @@ mod tests {
         });
     }
 
+    const REPETITIONS: u64 = 10_000;
+
+    #[bench]
+    fn bench_push_str_one_byte(b: &mut Bencher) {
+        b.bytes = REPETITIONS;
+        b.iter(|| {
+            let mut r = String::new();
+            for _ in range(0, REPETITIONS) {
+                r.push_str("a")
+            }
+        });
+    }
+
+    #[bench]
+    fn bench_push_char_one_byte(b: &mut Bencher) {
+        b.bytes = REPETITIONS;
+        b.iter(|| {
+            let mut r = String::new();
+            for _ in range(0, REPETITIONS) {
+                r.push('a')
+            }
+        });
+    }
+
+    #[bench]
+    fn bench_push_char_one_byte_with_fast_path(b: &mut Bencher) {
+        b.bytes = REPETITIONS;
+        b.iter(|| {
+            let mut r = String::new();
+            for _ in range(0, REPETITIONS) {
+                r.push_with_ascii_fast_path('a')
+            }
+        });
+    }
+
+    #[bench]
+    fn bench_push_char_two_bytes(b: &mut Bencher) {
+        b.bytes = REPETITIONS * 2;
+        b.iter(|| {
+            let mut r = String::new();
+            for _ in range(0, REPETITIONS) {
+                r.push('â')
+            }
+        });
+    }
+
+    #[bench]
+    fn bench_push_char_two_bytes_with_slow_path(b: &mut Bencher) {
+        b.bytes = REPETITIONS * 2;
+        b.iter(|| {
+            let mut r = String::new();
+            for _ in range(0, REPETITIONS) {
+                r.push_with_ascii_fast_path('â')
+            }
+        });
+    }
+
     #[bench]
     fn from_utf8_lossy_100_ascii(b: &mut Bencher) {
         let s = b"Hello there, the quick brown fox jumped over the lazy dog! \