Auto merge of #451 - JustForFun88:speed_up_resize_inner, r=Amanieu

Slightly speed up the `resize_inner` function + documentation of other functions.

This speeds up the `resize_inner` function a bit, since now reading the data from the heap is done not by byte, but by a group. In addition, we may not iterate over all control bytes if we have yielded all indexes of full buckets. For example, on my computer:

Before (with `cargo bench`):
```
test grow_insert_ahash_highbits  ... bench:      32,481 ns/iter (+/- 5,315)
test grow_insert_ahash_random    ... bench:      30,122 ns/iter (+/- 5,568)
test grow_insert_ahash_serial    ... bench:      34,267 ns/iter (+/- 8,364)
test grow_insert_std_highbits    ... bench:      54,410 ns/iter (+/- 5,030)
test grow_insert_std_random      ... bench:      52,566 ns/iter (+/- 6,455)
test grow_insert_std_serial      ... bench:      51,681 ns/iter (+/- 9,595)
```

After (with `cargo bench`):
```
test grow_insert_ahash_highbits  ... bench:      27,597 ns/iter (+/- 1,436)
test grow_insert_ahash_random    ... bench:      27,096 ns/iter (+/- 203)
test grow_insert_ahash_serial    ... bench:      27,369 ns/iter (+/- 719)
test grow_insert_std_highbits    ... bench:      50,399 ns/iter (+/- 449)
test grow_insert_std_random      ... bench:      44,495 ns/iter (+/- 7,180)
test grow_insert_std_serial      ... bench:      47,735 ns/iter (+/- 7,598)
```

As for the documentation, I started with `resize_inner` and since this function depends on others, I had to document them as well, etc., so there a lot of documentation in total. Also fixed a couple of inaccuracies with marking the function as unsafe.

Fix #453.

Author: bors