Inline pow2, remove a few proof asserts

Improved proof assertions (now depends less on things like reg54)

Remove or contract a few assertions, fixes slow xor assert verifications

... which is not good.

a version based on range types; this produces larger smt2 input but seems to perform quite okay on schoolbook

added a coercion; disabled the type invariant; other minor changes

See merge request !1

This is needed to prove B.Sum_def_empty and B.Sum_def_non_empty

Session isn't updated as the ocaml installation my laptop is a bit wonky and wont save properly