Improve the treatment of 'seq' (Trac #2273)
Trac #2273 showed a case in which 'seq' didn't cure the space leak
it was supposed to. This patch does two things to help
a) It removes a now-redundant special case in Simplify, which
switched off the case-binder-swap in the early stages. This
isn't necessary any more because FloatOut has improved since
the Simplify code was written. And switching off the binder-swap
is harmful for seq.
However fix (a) is a bit fragile, so I did (b) too:
b) Desugar 'seq' specially. See Note [Desugaring seq (2)] in DsUtils
This isn't very robust either, since it's defeated by abstraction,
but that's not something GHC can fix; the programmer should use
a let! instead.