1) Fix a bad bug in Subst.lhs that made uniqAway go into an
   infinite loop when the 'hash code' in the in-scope set was
   zero.

2) Rename BasicTypes.isFragileOccInfo to isFragileOcc
   Add isDeadOcc to BasisTypes


(2) is just a tidy-up.  I have to commit it now because of (1), which
is a bad bug.  I hope that I've committed all the files involved in (2).