Fix context reduction stack overflow in dimensional

[Bas van Dijk]

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
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