* goops.scm (compute-getters-n-setters/verify-accessors): Better

* goops.scm (compute-getters-n-setters/verify-accessors): Better
check of format of value returned by compute-get-n-set.
(compute-getters-n-setters): Extended format of slot
getters-n-setters to indicate position and size of slot memory
allocated in instances.

* goops.c (scm_sys_prep_layout_x): Instance allocation is now
indicated through extra fields in getters-n-setters.
(scm_add_slot): Adapted to new format of getters_n_setters slot.
(Thanks to Andy Wingo.)