Optimize Bytes::copy_from_slice

[stepancheg]

Create a new SharedInline Bytes representation, which is:

struct SharedInline {
    ref_cnt: AtomicUsize,
    cap: usize,
    // data: [u8; cap],
}

The advantage of this representation is that we do not need an extra allocation when cloning Bytes which makes such cloning much faster and without extra allocation.

The drawback is a slightly lower performance of such object destruction due to atomic decrement.

The bench:

#[bench]
fn copy_from_slice_and_clone(b: &mut Bencher) {
    b.iter(|| {
        Bytes::copy_from_slice(b"abcdef").clone()
    });
}

becomes two times faster (210ns/iter vs 110ns/iter).

While non-shared bench:

#[bench]
fn copy_from_slice(b: &mut Bencher) {
    b.iter(|| {
        Bytes::copy_from_slice(b"abcdef")
    });
}

becomes a little slower (85ns/iter vs 90ns/iter).