Implement class redefinition on top of fixed structs

* libguile/struct.h: Steal another flag for GOOPS.
* libguile/goops.h (SCM_VTABLE_FLAG_GOOPS_INDIRECT)
  (SCM_VTABLE_FLAG_GOOPS_NEEDS_MIGRATION): New flags.
  (SCM_CLASSF_GOOPS_VALID, SCM_CLASSF_GOOPS_OR_VALID): Remove obsolete
  definitions.
  (SCM_IS_A_P): Use the scm_class_of function.
* libguile/goops.c (var_class_of_obsolete_indirect_instance): Rename
  from var_migrate_instance.
  (scm_is_generic, scm_is_method, scm_sys_init_layout_x): Use
  scm_class_of instead of the SCM_CLASS_OF macro.
  (get_indirect_slots): New helper.
  (scm_class_of): This patch moves us in a direction where we won't be
  able to separately address a struct's data and its identity.
  Therefore to check whether a class needs migration, we check an
  embedded pointer from a slot instead of the vtable data.
  (scm_sys_struct_data): Remove this temporary function.
  (scm_sys_modify_instance): Update to swap slot values instead of the
  data pointers themselves.
  (scm_sys_modify_class): Use scm_sys_modify_instance.
  (scm_sys_goops_loaded): Capture class-of-obsolete-indirect-instance
  instead of migrate-instance.
  (scm_init_goops_builtins): Don't export the "valid" flag any more;
  export instead the "indirect" and "needs-migration" flags.
* libguile/foreign-object.c (scm_assert_foreign_object_type): Add a
  FIXME.
* libguile/vm-engine.c (class-of): Take away fast path for the time
  being.
* module/oop/goops.scm (class-has-indirect-instances?)
  (indirect-slots-need-migration?): New helpers.
  (<class>, <slot>, %class-slot-definition, initialize): Remove use of
  vtable-flag-goops-valid.
  (define-class): Always push redefined values through
  `class-redefinition'.
  (<redefinable-class>): New public definition.  Use it as a metaclass
  for redefinable classes.  Provide a compute-slots function that
  declares the indirect slots mechanism.  Add the "indirect" flag to
  instances of <redefinable-class>.  Create indirect-slots objects for
  instances of those classes as part of their allocate-instance.
  (change-object-class, class-of-obsolete-indirect-instance): Update for
  new representation change.
* test-suite/tests/goops.test ("object update"): Add #:metaclass
  <redefinable-class> to all redefinable classes.  For the "hell" test,
  make the new classes with class-direct-slots, not class-slots; this
  was an error in the test.

libguile/vm-engine.c

@@ -2905,8 +2905,7 @@ VM_NAME (scm_i_thread *thread, struct scm_vm *vp,
   VM_DEFINE_OP (113, class_of, "class-of", OP1 (X8_S12_S12) | OP_DST)
     {
       ARGS1 (obj);
-      if (SCM_INSTANCEP (obj))
-        RETURN (SCM_CLASS_OF (obj));
+      /* FIXME: restore fast path for direct instances.  */
       RETURN_EXP (scm_class_of (obj));
     }