This allows a 'Name:' to appear in front of an iface declaration's
name, which will cause 'Name' to refer to the self type (with the same
number of type parameters as the iface has) in the method signatures
of the iface. For example:
iface F: functor<A> {
fn fmap<B>(f: fn(A) -> B) -> F<B>;
}
Issue #1718