Optimize shallow_clone for Bytes::split_{off,to} (#92)

If `shallow_clone` is called with `&mut self`, and `Bytes` contains
`Vec`, then expensive CAS can be avoided, because no other thread
have references to this `Bytes` object.

Bench `split_off_and_drop` difference:

Before the diff:

```
test split_off_and_drop             ... bench:      91,858 ns/iter (+/- 17,401)
```

With the diff:

```
test split_off_and_drop             ... bench:      81,162 ns/iter (+/- 17,603)
```