0.9.15.3:

0.9.15.3:
	Implement the READER-METHOD-CLASS/WRITER-METHOD-CLASS protocol.

	In the process, note that the accessor methods generated for
	(slot-value x 'a) [ on generic functions of names like
	(SB-PCL::SLOT-ACCESSOR :GLOBAL A SB-PCL::READER) ] are not
	standard accessor methods, as they do not correspond to a given
	slot definition.  So implement
	GLOBAL-{READER,WRITER,BOUNDP}-METHOD classes for those, which
	have a slot name but no slot definition.

	Some rearrangements of early methods to support the new
	functionality.  REAL-MAKE-A-METHOD has to work moderately hard
	to separate out all the various ways it can be called.

	Include a test file for two ways of overriding the default
	methods.