Substantial improvements in RtClosureInspect

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