1677: Associated types r=flodiebold a=flodiebold
This implements basic support for (fully qualified) associated type projections:
- handle fully qualified paths like `<T as Trait>::AssocType` (basically desugaring to something like `Trait<Self=T>::AssocType`)
- lower these to a new `Ty::Projection` enum variant
- also introduce `Ty::UnselectedProjection` for cases like `T::AssocType` where the trait from which the type comes isn't specified, but these aren't handled further so far
- in inference, normalize these projections using Chalk: basically, when encountering a type e.g. from a type annotation or signature, we replace these `Ty::Projection`s by type variables and add obligations to normalize the associated type
Co-authored-by: Florian Diebold <flodiebold@gmail.com>