Refactor the treatment of lexically-scoped type variables for instance declarations
Previously the univerally-quantified variables of the DFun were also (bizarrely) used as the lexically-scoped variables of the instance declaration. So, for example, the DFun's type could not be alpha-renamed. This was an odd restriction, which has bitten me several times. This patch does the Right Thing, by adding an ib_tyvars field to the InstBindings record, which captures the lexically scoped variables. Easy, robust, nice. (I think this record probably didn't exist originally, hence the hack.)
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