Simplify HashMap Bucket interface
> Simplify HashMap Bucket interface
>
> * Store capacity_mask instead of capacity
> * Move bucket index into RawBucket
> * Valid bucket index is now always within [0..table_capacity)
> * Simplify iterators by moving logic into RawBuckets
> * Clone RawTable using RawBucket
> * Make retain aware of the number of elements
The idea was to put idx in RawBucket instead of the other Bucket types and simplify next() and prev() as much as possible. The rest was a side-effect of that change, except maybe the last 2.
This change makes iteration and other next/prev() heavy operations noticeably faster. Clone is way faster.
```
➜ hashmap2 git:(adapt) ✗ cargo benchcmp pre:: adp:: bench.txt
name pre:: ns/iter adp:: ns/iter diff ns/iter diff %
clone_10_000 74,364 39,736 -34,628 -46.57%
grow_100_000 8,343,553 8,233,785 -109,768 -1.32%
grow_10_000 817,825 723,958 -93,867 -11.48%
grow_big_value_100_000 18,418,979 17,906,186 -512,793 -2.78%
grow_big_value_10_000 1,219,242 1,103,334 -115,908 -9.51%
insert_1000 74,546 58,343 -16,203 -21.74%
insert_100_000 6,743,770 6,238,017 -505,753 -7.50%
insert_10_000 798,079 719,123 -78,956 -9.89%
insert_1_000_000 275,215,605 266,975,875 -8,239,730 -2.99%
insert_int_bigvalue_10_000 1,517,387 1,419,838 -97,549 -6.43%
insert_str_10_000 316,179 278,896 -37,283 -11.79%
insert_string_10_000 770,927 747,449 -23,478 -3.05%
iter_keys_100_000 386,099 333,104 -52,995 -13.73%
iterate_100_000 387,320 355,707 -31,613 -8.16%
lookup_100_000 206,757 193,063 -13,694 -6.62%
lookup_100_000_unif 219,366 193,180 -26,186 -11.94%
lookup_1_000_000 206,456 205,716 -740 -0.36%
lookup_1_000_000_unif 659,934 629,659 -30,275 -4.59%
lru_sim 20,194,334 18,442,149 -1,752,185 -8.68%
merge_shuffle 1,168,044 1,063,055 -104,989 -8.99%
```
Note 2: I may have messed up porting the diff, let's see what CI says.
327b9be9e9