Port WebRtc.PeerConnection to freedom-0.6's core.rtcpeerconnection.

This is a very rough draft with regard to code style, but it does
include a version of simple-freedom-chat that uses the new
WebRtc.PeerConnection.  This is working in Chrome, but not yet in
Firefox.
I don't feel strongly about this at all but just to mention that `rtcdatachannel.d.ts` would also be a fine filename (we haven't prefixed any of the other typings with `core-`).