Registered Hooks (cw0) should have a "kind"

[ethanfrey]

Right now we just store an address. This assumes that there is exactly one type of hook registered for a given contract.

What if we have multiple types of events? eg. change membership and change admin? we could add some more info to track that and let the module decide. I guess it should be able to list all hooks given the event - could store as a key in a map, or multiple maps... we probably need to refactor that module a bit in either case.