Reduce use of instances in hs-boot files

Several things here

* GHC no longer allows user-written Typeable instances,
  so remove them from hs-boot files.

* Generally, reduce the use of instances in hs-boot files. They are
  hard to track.  Mainly this involves using pprType, pprKind etc
  instead of just ppr.  There were a lot of instances in hs-boot
  files that weren't needed at all.

* Take TyThing out of Eq; it was used in exactly one place (in
  InteractiveEval), and equality is too big a hammer for that.