Member binding operators

[josh11b]

Define the binding operation used to compute the result of x.y, p->y, x.(C.y), and p->(C.y) as calling a method from user-implementable interfaces.

[jonmeow]

I think there's good detail here on ValueBind and RefBind. I do have several questions about extend api; it's not clear to me what the implementation of this would look like, so I'd like to have more specifics.

[zygoloid]

Looks good to me; I think this is ready to go ahead. There's one thing I'd like double-checked here, and one more coherence concern, but I think the latter is pre-existing so shouldn't block this proposal.

[zygoloid]

Looks good, thanks!

[chandlerc]

LGTM as well!!!