The main changes are (there are some other small changes):
- Using a specific type for trait IDs in the new solver, allowing us to simplify a lot of code.
- Add `BoundConst` similar to `BoundTy` and `BoundRegion` (previously consts used `BoundVar` directly), due to a new trait requirement.
To the extent possible.
Previously they were confused. Sometimes generic params were treated as `Param` and sometimes as `Placeholder`. A completely redundant (in the new solver) mapping of salsa::Id to ints to intern some info where we could just store it uninterned (not in Chalk though, for some weird reason).
Plus fix a cute bug in closure substitution that was caught by the assertions of Chalk but the next solver did not have such assertions. Do we need more assertions?
