DEFVST

[JONL at MIT-MC (Jon L White)]

Finally looks like the new DEFVST is a winner. some features:
1) All selector macros share same code;  also, all constructor
   macros share too.  
2) Output of DEFVST macro is primarily a setup of a call to an internal
   initializatino function; output of cons-a-FOO is also minimal.
3) STRUCT-TYPEP uses the typep slot of the class object, thereby
   saving one slot which used to be in each instance.
4) Integrates smoothly with the CLASS system, whether or not it is
   initially loaded (by constructing up "skeletal" classes for the
   three big important ones, and keeping a list of skeletons which
   need to be "fleshed-out" when EXTEND is loaded.