`deriveEsqueletoRecord` should generate a `ToMaybe` instance

[evanrelf]

For example, if I have this data type:

data Person = Person
  { name :: Text
  , age :: Natural
  }

If I invoke deriveEsqueletoRecord, I'll get a new SqlPerson data type, but it's missing a ToMaybe instance, which seems to be necessary for using the type in left joins.