Investigate potential removal of BackendKey

BackendKey adds a decent amount of complexity for questionable gain.

-- | 'ToBackendKey' converts a 'PersistEntity' 'Key' into a 'BackendKey'
-- This can be used by each backend to convert between a 'Key' and a plain
-- Haskell type. For Sql, that is done with 'toSqlKey' and 'fromSqlKey'.
--
-- By default, a 'PersistEntity' uses the default 'BackendKey' for its Key
-- and is an instance of ToBackendKey
--
-- A 'Key' that instead uses a custom type will not be an instance of
-- 'ToBackendKey'.
class ( PersistEntity record
      , PersistEntityBackend record ~ backend
      , PersistCore backend
      ) => ToBackendKey backend record where
    toBackendKey   :: Key record -> BackendKey backend
    fromBackendKey :: BackendKey backend -> Key record

class PersistCore backend where
    data BackendKey backend

The functions toSqlKey and fromSqlKey rely on it:

toSqlKey :: ToBackendKey SqlBackend record => Int64 -> Key record
toSqlKey = fromBackendKey . SqlBackendKey

fromSqlKey :: ToBackendKey SqlBackend record => Key record -> Int64
fromSqlKey = unSqlBackendKey . toBackendKey

These functions are primarily useful for turning the "default" backend key into a Key record polymorphically, so you can write get (toSqlKey 3) conveniently without needing to write get (UserKey 3).

Related to #352

Start by tracing the ToBackendKey and PersistCore definitions and the toSqlKey and fromSqlKey functions. Review related issue #352 and identify the API and backend implications of removing BackendKey; done means reaching and documenting a clear decision and its required follow-up changes.