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	Template-Haskell fix to make the global environment
		      more side-effect-ful
	---------------------------------------------------

Consider

	f = $(...foldl...) $(...foldl...)

The first splice sucks in the type sig for foldl, which the second
splice needs.  That means that the second splice is going to have to
consult the persistent compiler state to see the effect of imports
by the first one.

We used to cache the global type environment in the TcGblEnv, but
this commit switches to the obvious thing: consult the persistent
state on every global lookup.  After all, reading a MutVar is no
big deal; and it's a benign, ever-growing cache of type signatures,
so the side effect is fine.

On the way I tidied up the knot-tying in TcIfaceSig a bit more.
Previously, I think the setUnfoldingInfo was being strict in the
unfolding, which forced it to be type-checked.  Now it's lazy.
That could mean a lot less typechecking overall, for things whose
unfolding isn't looked at.  I hope I havn't broken it, though.