Fix #313 by doing some list munging.

I get rid of the Monoid instance because we weren't satisfying the laws.
Convenience of having <> didn't outweigh the shock-factor of having it
behave badly.
Stop testing #188.

Because the change is in GHC 7.9 and we now work against 7.8.3, this
test no longer makes sense. We revert it until 7.10 becomes the standard
version. If anything, there should be a test for this in GHC itself.