Extend defaulting plugin API to allow defaulting on multi-parameter type classes

Currently, the defaulting plugin API matches exactly the signature of the built-in defaulting mechanism, where each type variable occuring in the Wanteds can be proposed to be defaulted to a priority list of types, one by one, with some extra constraints that need to hold after defaulting.

Since each new constraint is associated with a single defaultable type variable, this makes it impossible to propose defaults for multi-parameter typeclasses C a b, or even constraints with multiple as-yet-unsolved type variablesC (T a b).

However, it would not take much to extend the plugin API to support this use case. I have a prototype implementation at cactus/ghc@b73d407b that changes the API to this:

data DefaultingProposal
  = DefaultingProposal
    { deProposalTyVars :: [TcTyVar]
      -- ^ The type variable to default.
    , deProposalCandidates :: [[Type]]
      -- ^ Candidate types to default the type variable to.
    , deProposalCts :: [Ct]
      -- ^ The constraints against which defaults are checked.
    }

The types could be a bit tighter, because basically we want a rectangle of proposal candidates (all (equalLength deProposalTyVars) deProposalCandidates), but I don't know how that's usually done in the GHC codebase so for the prototype this shall do.