Improved CHAMP map

[Theodus]

This PR includes modifications to the persistent hash map which have also resulted in a fix for #2866. The improvements include removing the U8 level field from all nodes and applying full CHAMP compression to the entries at each node.

[Theodus]

The new benchmarks are showing a slight improvement on lookup, and a larger improvement for iteration. However, there is a concerning increase in update times.

Old HashMap:

Benchmark                                   mean            median   deviation  iterations
apply 0 size 1024                         132 ns            132 ns      ±0.56%     1000000
apply 4 size 1024                         154 ns            154 ns      ±0.38%     1000000
apply 17 size 1024                        146 ns            146 ns      ±0.78%     1000000
apply 21 size 1024                        167 ns            167 ns      ±0.36%     1000000
apply 24 size 1024                        205 ns            205 ns      ±0.63%      500000
apply 30 size 1024                        173 ns            174 ns      ±0.70%     1000000
insert 0 size 1024                        306 ns            307 ns      ±0.69%      500000
insert 4 size 1024                        390 ns            390 ns      ±1.29%      300000
insert 17 size 1024                       321 ns            321 ns      ±0.50%      500000
insert 21 size 1024                       403 ns            403 ns      ±0.66%      300000
insert 24 size 1024                       573 ns            573 ns      ±1.22%      200000
insert 30 size 1024                       401 ns            401 ns      ±0.59%      300000
update 0 size 1024                        314 ns            314 ns      ±0.57%      500000
update 4 size 1024                        402 ns            402 ns      ±0.78%      300000
update 17 size 1024                       333 ns            333 ns      ±0.45%      500000
update 21 size 1024                       405 ns            405 ns      ±0.60%      300000
update 24 size 1024                       564 ns            565 ns      ±0.50%      200000
update 30 size 1024                       418 ns            419 ns      ±0.57%      300000
iter size 32                             3971 ns           3976 ns      ±0.87%       30000
iter size 1024                         209640 ns         209686 ns      ±0.44%        1000
iter size 32768                      25381650 ns       25368571 ns      ±4.16%           5

New HashMap:

Benchmark                                   mean            median   deviation  iterations
apply 0 size 1024                         132 ns            130 ns      ±0.37%     1000000
apply 4 size 1024                         152 ns            152 ns      ±0.26%     1000000
apply 17 size 1024                        149 ns            149 ns      ±0.35%     1000000
apply 21 size 1024                        165 ns            165 ns      ±0.49%     1000000
apply 24 size 1024                        200 ns            200 ns      ±0.54%      500000
apply 30 size 1024                        166 ns            165 ns      ±0.90%     1000000
insert 0 size 1024                        440 ns            441 ns      ±0.81%      300000
insert 4 size 1024                        573 ns            573 ns      ±0.50%      300000
insert 17 size 1024                       460 ns            458 ns      ±1.50%      300000
insert 21 size 1024                       584 ns            583 ns      ±0.37%      200000
insert 24 size 1024                       795 ns            795 ns      ±0.45%      200000
insert 30 size 1024                       516 ns            516 ns      ±0.49%      300000
update 0 size 1024                        435 ns            436 ns      ±0.74%      300000
update 4 size 1024                        503 ns            503 ns      ±0.63%      300000
update 17 size 1024                       464 ns            463 ns      ±1.26%      300000
update 21 size 1024                       539 ns            536 ns      ±1.27%      300000
update 24 size 1024                       689 ns            690 ns      ±0.69%      200000
update 30 size 1024                       541 ns            541 ns      ±0.57%      300000
iter size 32                             4267 ns           4273 ns      ±0.71%       30000
iter size 1024                         193530 ns         193667 ns      ±0.47%        1000
iter size 32768                      18047602 ns       17822185 ns      ±3.75%          10

[SeanTAllen]

@Theodus do the tests you are doing specifically prove that the bug has been fixed? If not, can you add a test that specifically covers #2866?

As this does more than 1 thing, can you add a change log entry to both Fixed and Changed?

[Theodus]

@SeanTAllen done and done

[SeanTAllen]

Merged!

[SeanTAllen]

@Theodus can you add release notes for this?

[mfelsche]

It should be noted that this also affects the persistent Set as it is based on HashMap

[Theodus]

Realease Notes

The persistent HashMap and HashSet have reduced memory consumption and improved iteration performance. This change also includes a bug fix related to searching for nonexistent entries and incorrect CHAMP compression.

@SeanTAllen