Add nfeu to iset.mm
Add nfeu to iset.mm

A few more existential uniqueness theorems and modfications
to make iset.mm catch up to set.mm.
- rename sb8 to sb8h and sb8e to sb8eh
- rename sb8eu to sb8euh
- add sb8 and sb8e which use F/ and match what is in set.mm
- add nfeu1, nfmo1, sb8eu, sb8mo (verbatim from set.mm)
- add nfeuv (lemma for nfeu)
- add nfeu (proved in terms of nfeuv, with no ax-3 usage)

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