add std.typecons.Typedef.toHash()

It doesn't appear as if `Proxy` was ever meant to be mixed into a class.
At least other operators don't handle that, see [opEquals](https://github.com/garlic-flavor/phobos/blob/master/std/typecons.d#L4092) and [opCmp](https://github.com/garlic-flavor/phobos/blob/master/std/typecons.d#L4104) for example.