Forward impl declaration of an incomplete interface

[josh11b]

Revise rules for what is required and provided by declarations and definitions of interfaces and impls. In particular:

• allow impl declarations of incomplete interfaces, and
• shift from a "use the information from the type definition if it happens to be complete" model to a "only use the information from the definition in contexts where it is required to be defined or complete" model.

Resolves questions-for-leads issues #4566, #4672, #4579.