Note [Finalising boxity for demand signatures]

The worker/wrapper pass must strictly adhere to the boxity decisions
encoded in the demand signature, because that is the information that
demand analysis propagates throughout the program. Failing to
implement the strategy laid out in the signature can result in
reboxing in unexpected places. Hence, we must completely anticipate
unboxing decisions during demand analysis and reflect these decisions
in demand annotations. That is the job of 'finaliseArgBoxities',
which is defined here and called from demand analysis.

Here is a list of different Notes it has to take care of:

  * Note [No lazy, Unboxed demands in demand signature] such as `L!P(L)` in
    general, but still allow Note [Unboxing evaluated arguments]
  * Note [No nested Unboxed inside Boxed in demand signature] such as `1P(1!L)`
  * Note [mkWWstr and unsafeCoerce]

NB: Then, the worker/wrapper blindly trusts the boxity info in the
demand signature; that is why 'canUnboxArg' does not look at
strictness -- it is redundant to do so.