Add a REPR for context references.

We don't make contexts have to be GCable, 6model objects. But if we
want to talk about them first class, they need objectifying. This will
provide that wrapping.
Implement nqp::ctx, nqp::ctxouter, nqp::ctxcaller.
Implement nqp::curcode() and nqp::callercode().
Update ROADMAP.