Add intuitionized mosub to iset.mm
Add intuitionized mosub to iset.mm

The theorem as stated in set.mm can be proved using the same
trick we used for mosubopt . Prove a closed form of mosub (which
we call mosubt ) just because that seems like good practice.

This replaces mosubex which can easily be removed as it is unused.

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