handle non-standard slot allocations when updating classes

handle non-standard slot allocations when updating classes

 * Refactor layout comparison to work on the effective slot definition list(s)
   directly -- easier to understand.

 * When new slots with custom allocation are added, add their names to the
   "added" list for UPDATE-INSTANCE-FOR-REDEFINED-CLASS. This is not specified
   by ANSI, but unless we do this those slots don't get initialized.

   Removing custom slots is hairier, as is changing a custom slot into
   a normal slot. Add some tests that poke in this area as well...

 * Replace wrapper-instance-slot-layout and wrapper-class-slots with the
   CLASS-SLOTS lists -- saves space and makes things easier to understand.

   Has a small performance cost for updating instances and SLOT-MISSING. Will
   refactor again if this is critical in the real world.