Add benchmarks and cleanup AMT

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Half of Add benchmarks to AMT/HAMT #202 and cleaned up some code I skimmed across (was written a while ago) but didn't dive too deep into optimizing

Benchmark diff from changes in PR:

AMT bulk insert (no flush)
                        time:   [29.919 us 30.224 us 30.730 us]
                        change: [-9.7947% -9.0350% -8.2663%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  4 (4.00%) high mild
  3 (3.00%) high severe

AMT bulk insert with flushing and loading
                        time:   [629.30 us 632.38 us 635.52 us]
                        change: [-25.441% -24.936% -24.448%] (p = 0.00 < 0.05)
                        Performance has improved.

AMT initialization from slice
                        time:   [76.709 us 77.116 us 77.501 us]
                        change: [-3.5445% -2.7607% -1.9730%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) high mild
  1 (1.00%) high severe

AMT for_each function   time:   [41.975 us 42.223 us 42.450 us]
                        change: [-3.1415% -2.3028% -1.5105%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) low mild
  1 (1.00%) high mild

Reference issue to close (if applicable)

Closes

Other information and links