reorganise proofs about hol's consistency

the proofs about the context extension rules are really more like
soundness rules; now they are in their own theory.

also, make a promise in the README (not yet fulfilled) to prove
HOL-without-infinity consistent (because this should be doable without
assumptions).

would also like to reduce the is_set_theory and is_infinite assumptions
to a single, self-contained, plausible-looking large-cardinal axiom.