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An INSTANCE is a vector-like object, perhaps of a new primitive type.  Slot
zero should contain either an instance-descriptor or another instance this is a
synonym for.  The other slots of the instance are used to store the state
variables for the object, either inline for efficiency or by pointing to other
structures.

An INSTANCE-DESCRIPTOR is a vector defstruct.  It has a SENDING-FUNCTION slot,
which should contain a function that takes arguments like (object message .
args).  Depending upon the implementation of SEND, this may also be any of
various distinguished values, signalling a built-in send function.  It also has
a TYPE slot, the value of which is returned by type-of.

It may be useful to be able to add state to an existing instance.  INSTANCE
RESIZING refers to the ability to restructure an instance to reflect a new
environment, and more specifically to the ability to conceptually grow an
instance's vector (adjustable vectors may not be used for efficiency in the
general case).

%Alloc-instance (size id &optional (initial-element <unbound>))
%Instance-ref (instance slot)
  Absolute (zero is the descriptor), does not follow synonyms.
%Instance-length (instance)
Get-self (instance)
  Follows the synonyms.
Instance-ref (instance slot)
  Uses %get-self.  Also perhaps checks for unbound ivs.
Iv-unbound-p (instance slot)
  Follows the synonyms.  This is primarily for initializing instances.
send (instance operation &rest args)
  sends a message to the instance.

Ideally there is no function call between the function doing the send and the
method invocation, but the flexibility of the sending function is there if
needed.

(typep obj 'instance) --> T, (instancep obj) --> T
(print obj) --> (send obj 'print stream depth)
(describe obj) --> (send obj 'describe)

This much is required for a serious implementation.  Exactly what other
features the kernel-kernel has are implementation-dependent.  The following are
suggestions.

(EVAL obj) --> (send obj 'EVAL)

Maybe dumping an instance should do a (send obj 'fasd-form) and eval the result
at loadtime to resurrect the object.  Probably (typep obj 'FROB) wants to do a
(send obj 'typep 'FROB) so it can work like defstruct.

A few systems will want to experiment with the notion of objects masquerading
as lisp types.  This is the crux of the argument for a function that can send
to *any* object-oriented system, so it can go into system code.