merge old and new style region constraints into one thing

[BoxyUwU]

currently the Infcx can store both old and new style constraints. This is kind of annoying from an implementation complexity POV. But also it means that it's easy to have logic written for old style region cosntraints which silently ignores new-style constraints which is wrong.

For example scrape_region_constraints in mir typeck seems to ignore abby's constraints I think? as well as helper methods like take_and_reset_region_constraints

We should only have one kind of region constraint and just use it everywhere even before abby is stable. This will need us to:

• Always canonicalize region constraints abby: always store nextgen region constraints in canonical form rust#161306
• Support "visible for leak check" in new style constraints support VisibleForLeakCheck #30
• Probably perf stuff too. all the logic for constructing abby constraints is not well optimized and may need fast paths for the "trivial cases".
• we have special "eq region" constraints which abby doesn't do https://doc.rust-lang.org/nightly/nightly-rustc/rustc_trait_selection/infer/canonical/ir/enum.RegionConstraint.html
• we need to fix UndoLog to not be ass for perf :3
• we store universes of region vars in the old style storage :3 unsure what to do here

#t-types/trait-system-refactor > `unic-ucd-name-0.9.0` minimization @ 💬

#t-types/assumptions-on-binders > merging old and new style constraints

[BoxyUwU]

unsure what to do about type outlives constraints :3