1.0.6.46: better standard-specializer-p implementation

1.0.6.46: better standard-specializer-p implementation
	Instead of defining a gf with several methods, define a new
	class STANDARD-SPECIALIZER to be used as a superclass to the
	various specializers that are handled internally in PCL.

	It's not exported, because we don't want people to subclass it:
	they will not be able to implement the relevant protocols.