New function: okToUnfoldInHiFile.

  - For values whose RHS have been deemed to be interface
    file unfolding candidates, do a last minute check to
    see whether the unfolding is really suitable.

    An unfolding is not suitable iff

       - contains a _casm_
       - contains a lit-lit

    The reason for not unfolding _casm_/lit-lits into
    interface files is that their C fragments are likely
    to mention #defines/functions that will be
    out-of-scope at an unfolding site. Turning off unfolding
    of such expressions avoid this unfortunate situation.