This patch collects a small raft of related changes

* Arrange that during 
     (a) rule matching and 
     (b) uses of exprIsConApp_maybe
  we "look through" unfoldings only if they are active
  in the phase. Doing this for (a) required a bit of 
  extra plumbing in the rule matching code, but I think
  it's worth it.

  One wrinkle is that even if inlining is off (in the 'gentle'
  phase of simplification) during rule matching we want to
  "look through" things with inlinings.  
   See SimplUtils.activeUnfInRule.

  This fixes a long-standing bug, where things that were
  supposed to be (say) NOINLINE, could still be poked into
  via exprIsConApp_maybe. 

* In the above cases, also check for (non-rule) loop breakers; 
  we never look through these.  This fixes a bug that could make
  the simplifier diverge (and did for Roman).  
  Test = simplCore/should_compile/dfun-loop

* Try harder not to choose a DFun as a loop breaker. This is 
  just a small adjustment in the OccurAnal scoring function

* In the scoring function in OccurAnal, look at the InlineRule
  unfolding (if there is one) not the actual RHS, beause the
  former is what'll be inlined.  

* Make the application of any function to dictionary arguments
  CONLIKE.  Thus (f d1 d2) is CONLIKE.  
  Encapsulated in CoreUtils.isExpandableApp
  Reason: see Note [Expandable overloadings] in CoreUtils

* Make case expressions seem slightly smaller in CoreUnfold.
  This reverses an unexpected consequences of charging for
  alternatives.

Refactorings
~~~~~~~~~~~~
* Signficantly refactor the data type for Unfolding (again). 
  The result is much nicer.  

* Add type synonym BasicTypes.CompilerPhase = Int
  and use it

Many of the files touched by this patch are simply knock-on
consequences of these two refactorings.