This commit moves the instance environment out of the Class data
structure, where it was immutable, to part of the type-checker
environment.  This change is absolutely essential as part of
our move to GHCi, and I think it's also going to be necessary
for Andrei's work on generic functions.

As part of this change, we can remove

  a) types/InstEnv.*	(thereby also removing a hi-boot loop)
  b) a tc-fixpoint-loop in TcModule

Both of these are worthwhile simplifications.