Remove mutability of the sets in class_handler_registry

It doesn't make sense, because the __getitem__ doesn't give you a particular
set, but rather a union of the relevant ones, so .add doesn't make sense.  So
just use frozenset, and enforce it. Now to add something, you must explicitly
union it with what's there.
Allow to set get_relationship in the initializer of ArgHandler

Use this to implement the Pow handlers for newask in terms of the new
handlers.

This ended up being not quite as pretty as I had hoped, so this might end up
changing.