Reduce boilerplate by using DefaultSignatures

[nikita-volkov]

E.g.:

{-# LANGUAGE DefaultSignatures #-}
class MonadLayer m => MonadLayerFunctor m where
  layerMap :: (forall b. Inner m b -> Inner m b) -> m a -> m a
  default layerMap :: MonadTransFunctor m => (forall b. Inner m b -> Inner m b) -> m a -> m a
  layerMap = transMap

This way having an instance of MonadTransFunctor, the user will be able to skip the layerMap = transMap part in the instance declaration of MonadLayerFunctor.