Desugar applications of 'seq' specially; fix Trac #1031

Desugar applications of 'seq' specially; fix Trac #1031

Merge to 6.6 branch.  Test case is dsrun014.

Note [Desugaring seq]  cf Trac #1031
~~~~~~~~~~~~~~~~~~~~~
   f x y = x `seq` (y `seq` (# x,y #))

The [CoreSyn let/app invariant] means that, other things being equal, because
the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus:

   f x y = case (y `seq` (# x,y #)) of v -> x `seq` v

But that is bad for two reasons:
  (a) we now evaluate y before x, and
  (b) we can't bind v to an unboxed pair

Seq is very, very special!  So we recognise it right here, and desugar to
	case x of _ -> case y of _ -> (# x,y #)

The special case would be valid for all calls to 'seq', but it's only *necessary*
for ones whose second argument has an unlifted type. So we only catch the latter
case here, to avoid unnecessary tests.