Fix context reduction stack overflow in dimensional

Dear Bjorn, Attached is a patch that fixes a context reduction stack overflow in your dimensional package. I noticed something weird though (that's why I'm CCing the ghc list). When I cabal build dimensional-0.8.2 I first get the context reduction stack overflow when I then build it again I get the error again (as expected). However when I try to build it for the third time it succeeds! Why is this? I attached the build log. Regards, Bas