Entirely re-jig the handling of default type-family instances (fixes Trac #9063)
In looking at Trac #9063 I decided to re-design the default instances for associated type synonyms. Previously it was all jolly complicated, to support generality that no one wanted, and was arguably undesirable. Specifically * The default instance for an associated type can have only type variables on the LHS. (Not type patterns.) * There can be at most one default instances declaration for each associated type. To achieve this I had to do a surprisingly large amount of refactoring of HsSyn, specifically to parameterise HsDecls.TyFamEqn over the type of the LHS patterns. That change in HsDecls has a (trivial) knock-on effect in Haddock, so this commit does a submodule update too. The net result is good though. The code is simpler; the language specification is simpler. Happy days. Trac #9263 and #9264 are thereby fixed as well.