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?
Item tree IDs are very unstable (adding an item of a kind invalidates all following items of the same kind). Instead use ast ids, which, since the previous commit, are pretty stable.
And make more queries non-interned.
Also flip the default for queries, now the default is to not intern and to intern a query you need to say `invoke_interned`.
This refactors how we deal with items in hir-def lowering.
- It now lowers all of them through an "ExpressionStore" (kind of a misnomer as this point) as their so called *Signatures.
- We now uniformly lower type AST into TypeRefs before type inference.
- Likewise, this moves macro expansion out of type inference, resulting in a single place where we do non-defmap macro expansion.
- Finally, this PR removes a lot of information from ItemTree, making the DefMap a lot less likely to be recomputed and have it only depend on actual early name resolution related information (not 100% true, we still have ADT fields in there but thats a follow up removal).
