The TypeRef name comes from IntelliJ days, where you often have both
type *syntax* as well as *semantical* representation of types in
scope. And naming both Type is confusing.
In rust-analyzer however, we use ast types as `ast::Type`, and have
many more semantic counterparts to ast types, so avoiding name clash
here is just confusing.
It improves compile time in `--release` mode quite a bit, it doesn't
really slow things down and, conceptually, it seems closer to what we
want the physical architecture to look like (we don't want to
monomorphise EVERYTHING in a single leaf crate).
The current system with AstIds has two primaraly drawbacks:
* It is possible to manufacture IDs out of thin air.
For example, it's possible to create IDs for items which are not
considered in CrateDefMap due to cfg. Or it is possible to mixup
structs and unions, because they share ID space.
* Getting the ID of a parent requires a secondary index.
Instead, the plan is to pursue the more traditional approach, where
each items stores the id of the parent declaration. This makes
`FromSource` more awkward, but also more correct: now, to get from an
AST to HIR, we first do this recursively for the parent item, and the
just search the children of the parent for the matching def
