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     Improve handling of lexically scoped type variables
	---------------------------------

If we have

	f :: T a -> a
	f (x :: T b) = ...

then the lexically scoped variable 'b' should refer to the rigid
type variable 'a', without any intervening wobbliness.  Previously
the in-scope type variables were always mutable TyVars, which were
instantatiated to point to the type they were bound to; but since
the advent of GADTs the intervening mutable type variable is a bad
thing.

Hence
  * In the type environment, ATyVar now carries a type
  * The call to refineTyVars in tc_pat on SigPatIn
    finds the types by matching
  * Then tcExtendTyVarEnv3 extends the type envt appropriately

Rater a lot of huff and puff, but it's quite natural for ATyVar
to contain a type.

Various other small nomenclature changes along the way.