This patch simplifies the code slightly, and simultaneously improves
full laziness by floating allocations (lambdas, constructor apps) out
of loops.

See Note [Escaping a value lambda] in SetLevels, which explains.

There's a test that shows it up: simplrun009

This relevant to SpecConstr, because a call looks like
	f lvl
instead of
	f (\x. blah)
and the latter is easier to match in a robust way.