Refactor: rewrite FastFixedSet

[Kroppeb]

I gotta fix the linenumbers in the tests

[skyrising]

We can rerun the checks if ed2d0caad527bae9d4ba01aa66c5e2f70d18ad42 is merged, then they probably pass.

[github-actions[bot]]

Test Results

     18 files  ±0       18 suites  ±0   1m 56s :stopwatch: ±0s 2 488 tests ±0  2 487 :heavy_check_mark: ±0  1 :zzz: ±0  0 :x: ±0  3 549 runs  ±0  3 546 :heavy_check_mark: ±0  3 :zzz: ±0  0 :x: ±0 

Results for commit 95d67b9a. ± Comparison against base commit 95d67b9a.

:recycle: This comment has been updated with latest results.