refactor: Clean up par_map a bit

[Veykril]

No description provided.

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[codspeed-hq]

CodSpeed Performance Report

Merging #742 will not alter performance

_{Comparing Veykril:veykril/push-rmymmuwytuzz (b9379b0) with master (8fb172b)}

Summary

✅ 12 untouched benchmarks

[Veykril]

@davidbarsky bad news, the current impl of par_map is unsound, we are smuggling the database reference across threads before calling clone on it.

[davidbarsky]

@davidbarsky bad news, the current impl of par_map is unsound, we are smuggling the database reference across threads before calling clone on it.

Crap, I'm hoping this is fixable.

[Veykril]

It is, we just need to eagerly fork_db, sending a Box<dyn Database> is fine, but &dyn Database is not given its not Send

[Veykril]

Fixed the issue and added some more tests for the other new APIs

[davidbarsky]

Changes look good to me. Should we add a par_for_each function as well?

[Veykril]

We could