RowDecoder

Must be implemented by the user of Magnolia to construct a typeclass for case class T using the provided type info. E.g. if we are deriving Show[T] typeclasses, and T is a case class Foo(...), we need to constuct Show[Foo].

This method is called 'join' because typically it will join together the typeclasses for all the parameters of the case class, into a single typeclass for the case class itself. The field CaseClass.params can provide useful information for doing this.