This patch adds a new kind of plugin, Hole fit plugins. These plugins
can change what candidates are considered when looking for valid hole
fits, and add hole fits of their own. The type of a plugin is relatively
simple,

```
type FitPlugin = TypedHole -> [HoleFit] -> TcM [HoleFit]
type CandPlugin = TypedHole -> [HoleFitCandidate] -> TcM [HoleFitCandidate]
data HoleFitPlugin = HoleFitPlugin { candPlugin :: CandPlugin
                                   , fitPlugin :: FitPlugin }

data TypedHole = TyH { tyHRelevantCts :: Cts
                       -- ^ Any relevant Cts to the hole
                     , tyHImplics :: [Implication]
                       -- ^ The nested implications of the hole with the
                       --   innermost implication first.
                     , tyHCt :: Maybe Ct
                       -- ^ The hole constraint itself, if available.
                     }

This allows users and plugin writers to interact with the candidates and
fits as th...