Properly handle normalization

Previously normalization was broken, which caused a lot of fake errors.

This fix most type mismatches of the new solver, and it also reverts many test regressions.

The downside is that now `chalk_ir::TyKind::AssociatedType`/`chalk_ir::TyKind::Alias` cannot be trusted anymore with their roles, namely: `AssociatedType` is always fully normalized and `Alias` only if it can possibly be normalized further. That seems okay as the new solver does not have this distinction at all (due to it being a lazy normalizer), so this will only hold for the migration time. This does mean we have to change some APIs, notably `hir::Type::walk()` and `TyFingerprint`, to treat `Alias` the same as `AssociatedType`.

Another small thing this commit does is to isolate processing of user-written types (currently involving replacing error types and normalizing, but in the future validation will also be needed) to separate functions.

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