Clarify main code paths in exhaustiveness checking
This PR massively clarifies the main code paths of exhaustiveness checking, by using the `Constructor` enum to a fuller extent. I've been itching to write it for more than a year, but the complexity of matching consts had prevented me. Behold a massive simplification :D.
This in particular removes a fair amount of duplication between various parts, localizes code into methods of relevant types when applicable, makes some implicit assumptions explicit, and overall improves legibility a lot (or so I hope). Additionally, after my changes undoing #76918 turned out to be a noticeable perf gain.
As usual I tried my best to make the commits self-contained and easy to follow. I've also tried to keep the code well-commented, but I tend to forget how complex this file is; I'm happy to clarify things as needed.
My measurements show good perf improvements on the two match-heavy benchmarks (-18.0% on `unicode_normalization-check`! :D); I'd like a perf run to check the overall impact.
r? `@varkor`
`@rustbot` modify labels: +A-exhaustiveness-checking
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