Adding Fallback Schema

[pablf]

/claim #584

Added a new Schema.Fallback[A, B] for a new sealed trait Fallback[A, B] with subclasses Left, Right and Both. The idea is to allow not only to decode from fallback semantics but to encode to them using just one schema.

The property fullDecode: Boolean in Schema.Fallback allows to completely retrieve a Fallback or to retrieve only the Left part from a Fallback.Both. So:

// with fullDecode = true
"""[3, "hello"]""" // decodes Fallback.Both(3, "hello")

// with fullDecode = false
"""[3, "hello"]""" // decodes Fallback.Left(3)

If it's not possible to decode the left member, it reads the right one:

// with a Schema.Fallback[Int, String]
"""["wrong", "right"]""" // decodes Fallback.Right("right")

(P.D. It seems like #588 is not possible to claim anymore)

[pablf]

Thanks :)