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This major patch implements the new OutsideIn constraint solving
algorithm in the typecheker, following our JFP paper "Modular type
inference with local assumptions".  

Done with major help from Dimitrios Vytiniotis and Brent Yorgey.

The main thing here is to allow us to provide type
signatures for 'deriving' bindings without pain.

Happy generates

      notHappyAtAll = error "Blah"

without a type signature, and currently the new
typechecker doesn't generalise it.  This patch
says "no monomorphism restriction" which makes it
generalise again.

Better would be to add a type sig to Happy's template

This avoids the name clash with the similar but
not identical CoreUtils.tryEtaReduce