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	Fix a rather obscure bug in tcGen
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This bug concerns deciding when a type variable "escapes",
and hence we can't generalise it.  Our new subsumption-checking
machinery for higher-ranked types requires a slightly
more general approach than I had before.  The main excitement
is in TcUnify.checkSigTyVars and its friends.

As usual, I moved functions around and cleaned things up a bit;
hence the multi-module commit.