GenericMethods should contain all MethodSpec-referenced methods,
including those which are non-generic methods inside generic types. This
patch adds those methods, and also implements parameter substitution for
type arguments in parameters & return types.
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.