Major refactoring of the type inference engine
This patch embodies many, many changes to the contraint solver, which make it simpler, more robust, and more beautiful. But it has taken me ages to get right. The forcing issue was some obscure programs involving recursive dictionaries, but these eventually led to a massive refactoring sweep. Main changes are: * No more "frozen errors" in the monad. Instead "insoluble constraints" are now part of the WantedConstraints type. * The WantedConstraint type is a product of bags, instead of (as before) a bag of sums. This eliminates a good deal of tagging and untagging. * This same WantedConstraints data type is used - As the way that constraints are gathered - As a field of an implication constraint - As both argument and result of solveWanted - As the argument to reportUnsolved * We do not generate any evidence for Derived constraints. They are purely there to allow "impovement" by unifying unification variables. * In consequence, nothing is ever *rewritten* by a Derived constraint. This removes, by construction, all the horrible potential recursive-dictionary loops that were making us tear our hair out. No more isGoodRecEv search either. Hurrah! * We add the superclass Derived constraints during canonicalisation, after checking for duplicates. So fewer superclass constraints are generated than before. * Skolem tc-tyvars no longer carry SkolemInfo. Instead, the SkolemInfo lives in the GivenLoc of the Implication, where it can be tidied, zonked, and substituted nicely. This alone is a major improvement. * Tidying is improved, so that we tend to get t1, t2, t3, rather than t1, t11, t111, etc Moreover, unification variables are always printed with a digit (thus a0, a1, etc), so that plain 'a' is available for a skolem arising from a type signature etc. In this way, (a) We quietly say which variables are unification variables, for those who know and care (b) Types tend to get printed as the user expects. If he writes f :: a -> a f = ...blah... then types involving 'a' get printed with 'a', rather than some tidied variant. * There are significant improvements in error messages, notably in the "Cannot deduce X from Y" messages.