370: Self params & type r=matklad a=flodiebold
This implements type inference for `self`, so field completion for methods taking `self` works now.
- rename `IMPL_ITEM` to `IMPL_BLOCK` -- rustc calls the methods etc. inside an impl `ImplItem`s, and the impl itself doesn't define an item, so I thought this name was clearer.
- add HIR for impl blocks -- we collect all impls in a crate at once, so we can go from methods to containing impls, and since we will later also need to find all impls for a certain type (which may be anywhere in the crate, I think?). We could be more lazy here, but I don't know if it's worth the complexity.
- resolve `self` and `Self` during type inference
- refactor a bit in ty.rs as well
Co-authored-by: Florian Diebold <flodiebold@gmail.com>
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