Introduce <slot> objects in GOOPS

Introduce <slot> objects in GOOPS

* module/oop/goops.scm (fold-class-slots): Change format to use proper
  slot specifications.
  (fold-slot-slots): Flesh out with all needed slots.
  (<class>): Update cons-layout to deal with new fold-class-slots form.
  Don't create slots; we do that later.
  (is-a?, get-keyword, *unbound, unbound?, %allocate-instance): Move
  definitions up.
  (<slot>, slot?): New definitions.
  (slot-definition-name, slot-definition-allocation)
  (slot-definition-init-keyword, slot-definition-init-form)
  (slot-definition-init-value, slot-definition-init-thunk)
  (slot-definition-options, slot-definition-getter)
  (slot-definition-setter, slot-definition-accessor)
  (slot-definition-slot-ref, slot-definition-slot-set!)
  (slot-definition-index, slot-definition-size): New definitions as
  accessors on <slot> objects.
  (class-slot-definition): Adapt to class-slots change.
  (direct-slot-definition-class, make-slot): New definitions.
  (make): Define a boot version that can allocate <slot> instances.
  (compute-direct-slot-definition)
  (compute-direct-slot-definition-initargs)
  (effective-slot-definition-class, compute-effective-slot-definition):
  New definitions.
  (build-slots-list): Adapt to slots being <slot> objects.
  (compute-get-n-set): New boot definition.
  (allocate-slots): New definition.  Replaces
  compute-getters-n-setters.
  (%compute-layout, %prep-layout): Adapt to changes.
  (make-standard-class): Make <slot> objects for direct-slots, and
  handle the allocate-slots protocol.
  (<foreign-slot>): Inherit from <slot>.
  (get-slot-value-using-name, set-slot-value-using-name!)
  (test-slot-existence): Adapt to using slot definition objects.
  (make-class): Allow slot specs or <slot> objects as the `slots'
  argument.
  (write): New method on <slot>.
  (class-slot-ref, class-slot-set!): Reimplement.
  (compute-slot-accessors, compute-getter-method)
  (compute-setter-method): Adapt to changes.
  (compute-getters-n-setters): Remove.  Yay!
  (compute-get-n-set): Adapt to use effective slot definitions instead
  of the getters-n-setters for #:class / #:each-subclass allocation.
  (%initialize-object): Adapt.
  (initialize): New method for <slot>.  Adapt method for <class>.

* module/oop/goops/active-slot.scm (compute-get-n-set):
* module/oop/goops/composite-slot.scm (compute-propagated-get-n-set):
  Use slot-definition-options to access options of slot.

* test-suite/tests/goops.test ("bad init-thunk"): Fix to be a "pass-if"
  instead of an "expect-fail".