This patch fixes TypeInfo.GetVTable so that it specializes the vtable
for generic type instances. Also fix a minor bug in PropertyInfo that
would pass null pointers to GetMethodByDefinition.
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.
