Add rewriting to typechecking plugins

Type-checking plugins can now directly rewrite type-families.
The TcPlugin record is given a new field, tcPluginRewrite.
The plugin specifies how to rewrite certain type-families with a value
of type `UniqFM TyCon TcPluginRewriter`, where:

type TcPluginRewriter
  =  RewriteEnv -- Rewriter environment
  -> [Ct]       -- Givens
  -> [TcType]   -- type family arguments
  -> TcPluginM TcPluginRewriteResult

data TcPluginRewriteResult
  = TcPluginNoRewrite
  | TcPluginRewriteTo
      { tcPluginRewriteTo    :: Reduction
      , tcRewriterNewWanteds :: [Ct]
      }

When rewriting an exactly-saturated type-family application,
GHC will first query type-checking plugins for possible rewritings
before proceeding.

Includes some changes to the TcPlugin API, e.g. removal
of the EvBindsVar parameter to the TcPluginM monad.