Add update method to Refined

[fthomas]

Something like this:

final case class Refined[T, P](get: T) extends AnyVal {
  def update(f: T => T)(implicit p: Predicate[P, T]): Either[String, Refined[T, P]]
}

[fthomas]

A better name and signature would probably be:

def mapRefine[U](f: T => U)(implicit p: Predicate[P, U]): Either[String, Refined[U, P]]

because it maps the T and then refines the resulting U.

And it should also be provided for @@ which hints at a type class that provides functions for refined values.