-------------------------------------
   Do the top-level tcSimpifyTop (to resolve monomorphic constraints)
   once for the whole program, rather than once per splice group
	-------------------------------------

This change makes the trivial program

	main = return ()

work again.  It had stopped working (emitting an error about Monad m
being unconstrained) because the 'checkMain' stuff (which knows special
things about 'main' was happening only *after* all the groups of
decls in the module had been dealt with and zonked (incl tcSimplifyTop).

Better to postpone.  A little more plumbing, but one fewer unexpected
happenings.