Generic instance for internal types

import System.Collections.Generic

class Person:
[property(Name)] _name = ""

l = List of Person()
l.Add(Person(Name: "Eric Idle"))
assert "Eric Idle" == l[0].Name

Currently this will trigger an internal compiler error in
ExternalGenericTypeDefinition.MakeGenericType. The solution is to
implement a new IType class, GenericTypeInstance, which can behave
like an instantiated generic type (this will also trigger need for
GenericMethodInstance, etc...).