swift234

Docs: add a "Tradeoffs" section to Objective-C Interoperability.

The new section is supposed to discuss the different models for various
pieces of infrastructure (messaging, methods, classes, subclassing, and
overriding) -- basically, how much of Swift is exposed to Objective-C.
I think the big issue right now is actually what I've labeled the
Messaging Model: whether arbitrary Swift-objects are id-compatible. That
basically has repercussions everywhere else.

This doc is kind of a mess right now, I know.

Swift SVN r3072