Fix the occurrence analyser

Ticket #18603 demonstrated that the occurrence analyser's
handling of

  local RULES for imported Ids

(which I now call IMP-RULES) was inadequate.  It led the simplifier
into an infnite loop by failing to label a binder as a loop breaker.

The main change in this commit is to treat IMP-RULES in a simple and
uniform way: as extra rules for the local binder.  See
  Note [IMP-RULES: local rules for imported functions]

This led to quite a bit of refactoring.  The result is still tricky,
but it's much better than before, and better documented I think.

Oh, and it fixes the bug.