Optimize Bytes::slice for short slices (#136)

Slice operation should return inline when possible

It is cheaper than atomic increment/decrement.

Before this patch:

```
test slice_avg_le_inline_from_arc   ... bench:      28,582 ns/iter (+/- 3,880)
test slice_empty                    ... bench:       8,797 ns/iter (+/- 1,325)
test slice_large_le_inline_from_arc ... bench:      27,684 ns/iter (+/- 5,920)
test slice_short_from_arc           ... bench:      27,439 ns/iter (+/- 5,783)
```

After this patch:

```
test slice_avg_le_inline_from_arc   ... bench:      18,872 ns/iter (+/- 2,937)
test slice_empty                    ... bench:       9,136 ns/iter (+/- 1,908)
test slice_large_le_inline_from_arc ... bench:      18,052 ns/iter (+/- 2,981)
test slice_short_from_arc           ... bench:      18,200 ns/iter (+/- 2,534)
```