** CONDITION-CLASS

(find-class 'warning) gives an object of type STRUCTURE-CLASS.
However, a WARNING is not a STRUCTURE-OBJECT, but a CONDITION-OBJECT,
which contradicts the requirement that instances of STRUCTURE-CLASS be
STRUCTURE-OBJECTs.  Fix this, probably by teaching PCL about
CONDITION-CLASS analogously to STRUCTURE-CLASS.

** CLASS-PROTOTYPE

(sb-pcl:class-prototype (find-class 'null)) yields something decidedly
weird -- it has allocated a NULL thingy.  This is easy to solve
[&optional (proto nil protop)]; probably harder are the issues for
e.g. BIGNUM, some strange array classes, and so on, particularly in
their interaction with the cross-compiler dumper, should PCL ever be
moved to the main build.

** SB-KERNEL

Now that CL:CLASS has turned into SB-KERNEL:CLASSOID, SB-PCL can
use-package SB-KERNEL.  This should have the pleasant effect of
ridding us of a fair amount of package prefix noise.

** SB-MOP

Now that a CL:CLASS is a real CLOS class, it might be worth giving
SB-PCL the SB-MOP nickname, since it now is in the region of
conforming to AMOP.