equivariant functions

[Christian Kamm]

Can you, given an alias to a function that uses this equivariance, create a
forwarder function with the exact same behaviour? I.e. will calls to

typeof(return) identity(alias fn)(ParameterTuple!(fn) args) {
  return fn(args);
}

or similar work exactly as calls to fn, even though typeof(return) has to be
more than just a type? The parameter storage classes ref, out, lazy already
make this kind of forwarding very tricky to do right (I think you have to
escape to mixins) and I fear this'll make it even more involved.