Deal with large extra-contraints wildcards
For reasons explained in TcHsType Note [Extra-constraint holes in partial type signatures], if we had f :: (_) => blahs and the '_' was filled in by more than a 62-tuple of contraints, GHC crashed. The same Note explains the hacky solution I have adopted to evade this. Maybe there is some better way, but I couldn't see one that didn't involve a great deal of work. And the problem is a very narrow one! If the hack bites us we'll need to think again.
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