Add mulcanenq to iset.mm
Add mulcanenq to iset.mm

The proof is taken from the 2012 set.mm proof of distrpqlem
but is fairly similar to the current set.mm proof of mulcanenq
in the sense that both of them are based on enqbreq and natural
number closure, commutativity, and associativity.

metamath/set.mm,metamath/set.mm,metamath/set.mm