Trying to fix an efficiency issue noted in a TODO in SAT.hs

compiler/simplCore/SAT.hs has a TODO comment about the fact that it does a fair bit of appending onto the ends of lists, and that should be done differently. I made an attempt to fix it. The complexity of the recursion, however, leaves me uncertain as to whether I really did or not. I've attached a diff and I hope someone will be able to take a look at it. The only use of Sequence.fromList is source line 172, and the only significant use of Foldable.toList (aside from pretty-printing) is on source line 402. Note that the use of Sequence may be temporary—I want to get the right code structure down before choosing the best data structure. Thanks, David Feuer