A clean-up pass on fundeps and implicit params.  Haven't yet incorporated
changes from Hugs/GHC meeting yet, tho.
  - Fixed up several places in Type.lhs where IPNotes were probably being
    incorrectly handled.  Strongly suggests a better solution should be
    implemented for marking implicit params than piggybacking on NoteTys.
  - tcSimplifyAndCheck was handling implicit params incorrectly
    (holding on to them when it should have been booting them out to frees).
  - Improved improvement WRT type signatures (the signature is now taken
    into account when improving).
  - Added improvement when matching against local polymorphic types.