Fix Trac #2130: improve derived Ord for primmitive types

Fix Trac #2130: improve derived Ord for primmitive types

This patch does two things:

* (Minor): in TcGenDeriv.careful_compare_Case, test for less-than before
  equality. This should reduce the number of dynamic tests, and also gives
  more scope for optimisation, since less-than tells us more than equality.

* (More important): add special-case derived code for data types that are
  simple wrappers of primitive types. See
	Note [Comparision of primitive types]
  This fixes Trac 2130.

However see also Trac #2132, which is not addressed here.