notably, this modifies the proof method `this`: the previous version
of it no longer works with cconv.ML (borrowed from HOL), so now it's
just a call to the simplifier, which does work.

Unfortunately the new `this` can otherwise do less than the old one
(it does not instantiate schematic variables), so the old one is now
available as `infer` instead.

1. Rudimentary prototype definitional package
2. Started univalence
3. Various compatibility fixes and new theory stubs
4. Updated ROOT file