Weaken superclass requirements

currently if we write

`[instances | Foo f => Bar (Baz f) where ...|]`

then all the methods will have the required context `Foo f`, even if that particular superclass does not require `Foo`, making our instances hold in less places than they should. This is remedied by manually adding a superclass instances with the correct context before `instances`, but in the worst case every superclass has a weaker context than the subclass so `instances` is useless.

We'd like to automatically weaken the context for each superclass based on the actual constraints on the default methods. This is likely to be tricky and possibly require implementing type inference.