Add HoTT book references to iset.mm
Add HoTT book references to iset.mm

This is definition 11.2.7(vi) which has ten statements under it. Four
of them - lenlt , 0lt1 , ltadd1 , and leadd1 - we already have, so just
add references to the HoTT book to the comments of those theorems.

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