Optionally derive PathMultiPiece for composite keys (#1509)

* Move derivation strategy choice to `where` clause

* Create PersistPathMultiPiece typeclass

* Create spec and derive PersistPathMultiPiece instance in dataTypeDec

* Revert changes to mkKeyTypeDec

This reverts part of commit 500feba.

* Fix and test instance derivation using mpsDeriveInstances

This reverts part of commit 7b9e76f.

* Move PersistPathMultiPiece into PersistEntity module

This avoids creating an orphan instance of PathMultiPiece.

* Document feature in Database.Persist.Quasi

* Add top-level `describe` to CompositeKeyPathMultiPieceSpec

* Only derive PathMultiPiece for composite keys

As mentioned in the MkPersistSettings Haddock change, specifying
PersistPathMultiPiece directly overrides this behaviour.

* Fix PersistPathMultiPiece override and add tests for it

* Use DerivingVia with newtype wrapper

This reverts commit 5bc5799.

* Only use DerivingVia on GHC versions with support for it

* Add CPP language pragma to spec

* Revert changes to Database.Persist.TH and remove associated spec

persistent/Database/Persist/Class/PersistEntity.hs

@@ -24,6 +24,7 @@ module Database.Persist.Class.PersistEntity
     , FilterValue (..)
     , BackendSpecificFilter
     , Entity (.., Entity, entityKey, entityVal)
+    , ViaPersistEntity (..)
 
     , recordName
     , entityValues
@@ -56,6 +57,7 @@ import Data.Aeson.Text (encodeToTextBuilder)
 import Data.Aeson.Types (Parser, Result(Error, Success))
 import Data.Attoparsec.ByteString (parseOnly)
 import Data.Functor.Identity
+import Web.PathPieces (PathMultiPiece(..), PathPiece(..))
 
 #if MIN_VERSION_aeson(2,0,0)
 import qualified Data.Aeson.KeyMap as AM
@@ -180,6 +182,18 @@ class ( PersistField (Key record), ToJSON (Key record), FromJSON (Key record)
     keyFromRecordM :: Maybe (record -> Key record)
     keyFromRecordM = Nothing
 
+-- | Newtype wrapper for optionally deriving typeclass instances on
+-- 'PersistEntity' keys.
+--
+-- @since 2.14.6.0
+newtype ViaPersistEntity record = ViaPersistEntity (Key record)
+
+instance PersistEntity record => PathMultiPiece (ViaPersistEntity record) where
+    fromPathMultiPiece pieces = do
+        Right key <- keyFromValues <$> mapM fromPathPiece pieces
+        pure $ ViaPersistEntity key
+    toPathMultiPiece (ViaPersistEntity key) = map toPathPiece $ keyToValues key
+
 -- | Construct an @'Entity' record@ by providing a value for each of the
 -- record's fields.
 --