Improve pretty-printing of types

In this commit
   commit 6c0f10fa
   Author: Ben Gamari <bgamari.foss@gmail.com>
   Date:   Sun Nov 13 16:17:37 2016 -0500

    Kill Type pretty-printer

we switched to pretty-printing a type by converting it to an
IfaceType and pretty printing that.  Very good.

This patch fixes two things

* The new story is terrible for debug-printing with -ddump-tc-trace,
  because all the extra info in an open type was discarded ty the
  conversion to IfaceType.

  This patch adds IfaceTcTyVar to IfaceType, to carry a TcTyVar in
  debug situations.  Quite an easy change, happily.  These things
  never show up in interface files.

* Now that we are going via IfaceType, it's essential to tidy before
  converting; otherwise
     forall k_23 k_34. blah
  is printed as
     forall k k. blah
  which is very unhelpful.  Again this only shows up in debug
  printing.