Make sense of the mess that were (are) different kind of generics in the solver

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?

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