This basically finishes the concrete generics implementation. We can now
enumerate all members of a concrete generic type with full type
substitution implemented.
Also add a simple test to verify that we can obtain the correct type for
a field of a concrete generic type.
We use indices into the TypesByReferenceIndex table to defer type lookup
during model construction. However, this won't support fully generic
instances (which might not be in the type table at all). This patch
introduces a new TypeRef abstraction which can either represent a fixed
index or an instantiated generic type.
