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This patch makes
	(s1~t1) == (s2~t2) iff s1==s2, t1==t2

The comparison and equality instances for Insts are really only
used in making the AvailEnv in TcSimplify, which equalities are
never put into, which is why we've gotten away with this for so long.

But I'm now using 'nub' in the error messages for equalities,
and in any case it makes sense to have the right equality!

In improving the specialiser's data reprsentaion, thus
  Mon Apr 28 16:57:11 GMT Daylight Time 2008  simonpj@microsoft.com
  * Fix Trac #1969: perfomance bug in the specialiser
I got the sense of a pair of filter functions back to front.
As a result, almost all specialisation opportunities were being
filtered out, and no specialisations were being generated.

Fortunately, dolio notice and reported Trac #2486.  The fix is
simple: put the filters the right way roud.

Spotted by Simon

If you "make clean" in the root then we still clean all stages

The Core preprocessor was rebuilding the type and data constructor environments every time it called the typechecker, which was horribly inefficient. Fixed.