0.8.12.28:

0.8.12.28:
	Better error messages for when the MOP instance structure
	protocol is violated (e.g. by the user defining a class with
	slots with non-standard :allocation, but no methods to go with
	it)
	... new :amop reference source;
	... new instance-structure-protocol-error condition.  Should
		probably eventually become a subclass of MOP-ERROR, once
		we start accumulating those;
	... move implementation of slot-valueish logic around a little
		to support these better error messages.

	(the ctor.lisp optimization is broken in the presence of
	non-standard slot allocation, and also in the presence of
	auxiliary methods on slot-value-using-classish generic
	functions.  Working on it...)