Substantial improvements in RtClosureInspect

The code for constructors in cvReconstrutType and cvObtainTerm
was grotesque.  It's now slightly better.  And it works with
the new coercion representation.

Apart from being simpler I have also made some small behavioural
improvements
  * Improved printing for string values
  * Preserve the print-name when instanting type variables
    (so now they are not always called "t")
  * Suppressed printing of predicate arguments to
    data constructors, unless -dppr-debug is on