Add liballoc impl SpecFromElem for i8
Speedup vec![1_i8; N] for non-zero element.
Before
test do_bench_from_elem_i8 ... bench: 130 ns/iter (+/- 7) = 61 MB/s
test do_bench_from_elem_u8 ... bench: 121 ns/iter (+/- 4) = 66 MB/s
After
test do_bench_from_elem_i8 ... bench: 123 ns/iter (+/- 7) = 65 MB/s
test do_bench_from_elem_u8 ... bench: 121 ns/iter (+/- 5) = 66 MB/s
No speed difference if element is already zero.
```rust
#[bench]
fn do_bench_from_elem_i8(b: &mut Bencher) {
b.bytes = 8 as u64;
b.iter(|| {
let dst = ve::vec![10_i8; 100];
assert_eq!(dst.len(), 100);
assert!(dst.iter().all(|x| *x == 10));
})
}
```
As suggested by @cuviper
https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/SpecForElem.20for.20other.20integers
r? @cuviper
CC @joshtriplett
Edit: Wow, I just realized both reviewers are Josh.