auto merge of #11061 : huonw/rust/opt-unsafe-vec, r=alexcrichton

Before:

```
test vec::bench::random_inserts                     ... bench:     15025 ns/iter (+/- 409)
test vec::bench::random_removes                     ... bench:     16063 ns/iter (+/- 276)
```

After:

```
test vec::bench::random_inserts                     ... bench:      5257 ns/iter (+/- 321)
test vec::bench::random_removes                     ... bench:      4980 ns/iter (+/- 94)
```

diff --cc src/libstd/vec.rs
Simple merge