Simplify WaitGroup implementation

[Veykril]

No description provided.

[netlify]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
🔨 Latest commit	9b4da36
🔍 Latest deploy log	https://app.netlify.com/projects/salsa-rs/deploys/68f610e0389171000802e4d7

[codspeed-hq]

CodSpeed Performance Report

Merging #958 will degrade performances by 5.28%

_{Comparing Veykril:veykril/push-rqyrkyttxpol (9b4da36) with master (9cfe41c)}

Summary

❌ 1 (👁 1) regression
✅ 12 untouched

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
👁	amortized[Input]	2.1 µs	2.2 µs	-5.28%

[MichaReiser]

Is there any other benefit than code size reduction. It seems codspeed doesn't like it at all.

[Veykril]

No there isn't. I was curious about the perf impact, I agree this isn't great. Checking if we can actually simplify our impl now

[Veykril]

Hm, we could get rid of the additional Arc as well and re-use the Zalsa arc but that makes the code a lot more ugly (thanks to having to deal with drop order), so this is the best I can come up with. Trading an unwrap with unsafe solely due to polonius borrow checking not being a thing yet :(

[MichaReiser]

@ibraheemdev could you take a look at this PR?

[ChayimFriedman2]

This introduced a potential to deadlock. I'm still investigating how exactly, but my guess is that the atomic ordering is not correct (it's not at least in one thing, but that doesn't fix the deadlock).