this just replaces all instance of `this` with instances of `infer`.
Unfortunately, it looks likes something else also broke, and I have
no idea what it is (but the proof for equiv_if_equal fails).

Sadly this means we can't get to univalence for now …
(but rn I'm too tired to try anything else with it)

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