replace Thread.h with C++ concurrency primitives

[convextriangle]

Resolves #89;
For double-checking / questions for review stuffs (things that I remembered):

• are OnsetThreads and FindOnsetIntervals / other old parallel implementations necessary?
• is it worth it to leave BackgroundThread as a inheritance-ish based std::jthread wrapper or does it need more refactorings (to, e.g., eliminate it)?

[sukibaby]

I would appreciate some proofs that this PR maintains expected behavior. I feel like I'm seeing a lot of stuff deleted that I would have expected to be refactored.

[convextriangle]

I would appreciate some proofs that this PR maintains expected behavior. I feel like I'm seeing a lot of stuff deleted that I would have expected to be refactored.

1. parallel execution (iirc) was dead code, uvcat suggested nuking it but i'll check on stuff
2. proof as in... a video of me trying stuff?

expected to be refactored

3. such as?
   p.s. i'm asking for clarification - i do have questions as mentioned in the pr and i do not know your expectations as you have not planned/discussed this if i recall correctly so please do @sukibaby

[uvcat7]

LGTM. There isn't performance impact on calculating offsets so looks like nothing will revert.