initial interfaces for matching rtc API

Add interfaces for ```core.rtcdatachannel``` and ```core.rtcpeerconnection```. Eventually we will likely also want a ```core.mediastream``` api to implement ```getLocalStreams``` and friends.
So, this is the only part of this pull request that makes me uneasy.

I see this interface purely as a way to get access to "privileged" APIs; you have additional goals.

That's fine but *please don't be surprised by a potentially large number of requests from me proposing small adjustments to these arguments and return types*.