In the previous commit about "aggressive primops" I wanted a new
function CoreUtils.exprCertainlyTerminates.  In doing this I ended up
with a significant refactoring in CoreUtils.  The new structure has
quite a lot of nice sharing:

         exprIsCheap             = exprIsCheap' isHNFApp
         exprIsExpandable        = exprIsCheap' isConLikeApp

         exprIsHNF               = exprIsHNFlike isHNFApp
         exprIsConLike           = exprIsHNFlike isConLikeApp
         exprCertainlyTerminates = exprIsHNFlike isTerminatingApp

This patch also does some renaming

    CheapAppFun      -->   FunAppAnalyser
    isCheapApp       -->   isHNFApp
    isExpandableApp  -->   isConLikeApp