Provide HasField instances for Entity for OverloadedRecordDot

[parsonsmatt]

OK, so this is a fun PR!

We can provide:

instance (SymbolToField sym rec typ, PersistEntity rec) => HasField sym (Entity rec) typ where
  getField =
    view (fieldLens (symbolToField @sym @rec @typ))

which uses similar machinery to this esqueleto PR.

It uses the non-prefixed versions of the fields, which means that we're also polymorphic over what this means.

mkPersist sqlSettings [persistLowerCase|
User
    name Text
    age Int
|]

main = do
    let user = Entity (toSqlKey 1) User { userName = "Matt", userAge = 33 }
    print user.name
    print user.age

This is a polymorphic entity use:

wow :: (PersistEntity rec, SymbolToField "hello" (Entity rec) a, Show a) => Entity rec -> IO ()
wow rec = print rec.hello 

Which does mimic the polymorphism in HasField generally.

---

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[hw202207]

Notes:
This implementation had been removed at later version: #1401