Auto merge of #125069 - amandasystems:scc-refactor, r=nikomatsakis

Extend SCC construction to enable extra functionality

Do YOU feel like your SCC construction doesn't do enough? Then I have a patch for you! SCCs can now do *everything*! Well, almost.

This patch has been extracted from #123720. It specifically enhances
`Sccs` to allow tracking arbitrary commutative properties (think min/max mappings on nodes vs arbitrary closures) of strongly connected components, including
- reachable values (max/min)
- SCC-internal values (max/min)

This helps with among other things universe computation. We can now identify
SCC universes as a reasonably straightforward "find max/min" operation during SCC construction. This is also included in this patch.

It's also more or less zero-cost; don't use the new features, don't pay for them.

This commit also vastly extends the documentation of the SCCs module, which I had a very hard time following. It may or may not have gotten easier to read for someone else.

I believe this logic can also be used in leak check, but haven't checked. Ha. ha. Ha.

Author: bors