Unclear how to use simple newtype wrappers in beam table columns

Related to #262.

I think it expands on that because I try something beyond what the recommendations there cover and use `just_` with my that custom column type:

```haskell
guard_' (just_ emails ==?. _userEmail users)
````

I tailored that from the example on [values support](https://tathougies.github.io/beam/user-guide/queries/select/#values-support).

I hope you don't mind, most of my commentary is inline comments in the Haskell code.

```haskell
data UserT f
= UserT { _userId :: Columnar f (SqlSerial Int)
, _userEmail :: Columnar f Email
}
type User = UserT Identity

textToEmail :: Text.Text -> Maybe Email
textToEmail = undefined

newtype Email = Email Text.Text

instance Beam.HasSqlEqualityCheck Beam.Postgres Email

instance Beam.FromBackendRow Beam.Postgres Email where
fromBackendRow = do
val <- Beam.fromBackendRow
case textToEmail val of
Right email -> pure email
Left str -> fail $ "invalid email: " <> str

-- that works until I want to do something like
-- NOTE assuming this is like
-- select * from users where user.email in ('email1@email.com', ...)
getUsersByEmail :: [Email] -> IO [User]
getUsersByEmail listOfEmails = runSelectReturningList @Beam.Postgres @Beam.Pg @User . select $ do
users <- all_ (ViewingFact._metricsViewingFact ViewingFact.metricsDb)
emails <- values_ listOfEmails
guard_' (just_ emails ==?. _userEmail users)
pure users
```

If time allows I'll make a repo of this example for others who run into this problem and if necessary for us to debug with.