Interface namespaces (#1287)

This implements namespaces for interface methods, as discussed in #1266.

In brief:
- each interface has a namespace
- interface method resolution uses a linearized (c4) search for
appropriate methods, bottoming at the unqualified method name (for UX
and backwards compatibility)
- it becomes undesirable (and not easy) to call a namespace method
dynamically; you really have to go through the interfrace -- and this is
the desirable and safe way to invoke an interface method, as the
contract is enforced. It is also now safe to assert the interface method
types in the method implementation as you really have to go out of your
way to bypass the contract.

Follow up:
- what is not implemented yet is to validate the method implementation
against the interface method signature and in fact inject the signature
into the method automatically. This has many benefits (both performance
and safety), but it is also more complicated to implement. We can tackle
this in a follow up pr.