Primitive E module for a kind of pass-gate style mux.
This is a special kind of mux that can preserve Z values on its inputs, used to support experimental esim decomposition.
Definition:
(defconst *esim-zif* '(:n *esim-zif* :x (:out ((|o| zif |sel| |a| |b|))) :i ((|sel|) (|a|) (|b|)) :o ((|o|))))