Make the HaskStrong type representation Kind-indexed, and many supporting changes...

Make the HaskStrong type representation Kind-indexed, and many supporting changes (see long comment).

This patch makes a whole lot of interlocking changes, with the
ultimate (accomplished) goal of changing the HaskStrong type
representation ("HaskType") so that it is indexed by the Haskell Kind
of the type.  As a result, the auxiliary well-kindedness judgment
\vdash_{ty} is no longer necessary.

Other changes in this patch:

  - Added Coq ToString class (similar to Haskell's Show class)
  - Massive reduction in volume of code extracted for string literals
  - Decidable equality on HaskStrong's
  - Much more reliable HaskWeakToStrong, although it has regressed
    a bit in terms of number of cases handled; this will be remediated
    shortly.