Investigate broken `if` type distribution in mtprng

[dgrnbrg]

Note that many of the mux2s all need to have their branches casted to (uintm 32), or else there are type errors in the mux2. It seems like mux2 should type-unify its branches, so that one could write

(mux2 (= counter max-value)
  0
  (inc counter))