* tests/goops.test: Added tests for class redefinition, object

* tests/goops.test: Added tests for class redefinition, object
update and active slots.

* goops.scm (compute-getter-method): For custom getter: Check
boundness even if there is an init-thunk.  (The getter can return
#<unbound> even if the slot has been set before.)
(remove-class-accessors!): Also remove accessor-method from its
accessor.

* goops.c (scm_sys_fast_slot_ref): Use SCM_SLOT instead of
scm_at_assert_bound_ref.  (We don't want the unbound check.  See
oop/goops/active-slot.scm.)