The GOOPS "unbound" value is a unique pair

The GOOPS "unbound" value is a unique pair

* libguile/goops.c (SCM_GOOPS_UNBOUND, SCM_GOOPS_UNBOUNDP): Remove
  internal macros.
  (scm_make_unbound, scm_unbound_p): Remove internal functions.
  (scm_sys_clear_fields_x): Add "unbound" parameter, for the init
  value.

* module/oop/goops.scm (*unbound*): Define in Scheme as a simple
  heap-allocated value.
  (unbound?): New definition.
  (%allocate-instance): Pass *unbound* to %clear-fields!.
  (make-class, slot-definition-init-value)
  (slot-definition-init-form, make-closure-variable): Use *unbound*
  instead of (make-unbound), which is now gone.

* module/oop/goops/active-slot.scm (compute-get-n-set): Use *unbound*
  instead of make-unbound.  This module uses the GOOPS internals module;
  perhaps we should export make-unbound or something...

* module/oop/goops/save.scm (make-unbound): Export our own make-unbound
  definition, for use by residualized save code.

* module/language/ecmascript/base.scm (<undefined>, *undefined*): Use a
  unique object kind and instance for the undefined value.

* libguile/vm.c (scm_i_vm_mark_stack): Fill the stack with
  SCM_UNSPECIFIED instead of SCM_UNBOUND.