This appears to be the primary cause of significant performance
regressions in the `prost` test suite in the 0.5 to 0.6 transition. See
danburkert/prost#381.
* Inner: make uninitialized construction explicit
* Remove Inner2
* Remove unnecessary transmutes
* Use AtomicPtr::get_mut where possible
* Some minor tweaks
I found this significantly improved a
[benchmark](https://gist.github.com/danburkert/34a7d6680d97bc86dca7f396eb8d0abf)
which calls `bytes_mut`, writes 1 byte, and advances the pointer with
`advance_mut` in a pretty tight loop. In particular, it seems to be the
inline annotation on `bytes_mut` which had the most effect. I also took
the opportunity to simplify the bounds checking in advance_mut.
before:
```
test encode_varint_small ... bench: 540 ns/iter (+/- 85) = 1481 MB/s
```
after:
```
test encode_varint_small ... bench: 422 ns/iter (+/- 24) = 1895 MB/s
```
As you can see, the variance is also significantly improved.
Interestingly, I tried to change the last statement in `bytes_mut` from
```
&mut slice::from_raw_parts_mut(ptr, cap)[len..]
```
to
```
slice::from_raw_parts_mut(ptr.offset(len as isize), cap - len)
```
but, this caused a very measurable perf regression (almost completely
negating the gains from marking bytes_mut inline).
