Update Value.scala

#example added why a ~value may be valuably implemented as type-classes, i.e. implemented as a class that provides ad-hoc polymorphism to values in scope and context;  the example holds already for more then a year in the scala collection implementation, the object ``Value`` containing a variable map of identifiers to the higher kinded Value[_] type;  indicating that a value requires ad-hoc polymorphism when it has to deal with other values.  i put a comment referring to my understanding of scala implementations employing scala's type-class pattern, feel free to implement.