Move GOOPS boot to Scheme

Move GOOPS boot to Scheme

* module/oop/goops.scm (build-<class>-slots): New helper, replacing
  build_class_class_slots.
  (build-slots-list, %compute-getters-n-setters, %compute-layout): New
  private helpers, moved here from C.
  (%prep-layout!): Reimplement in Scheme.
  (make-standard-class): New private helper, replacing
  scm_basic_make_class.
  (<class>, <top>, <object>): Define in Scheme.

  (<foreign-slot>, <protected-slot>, <hidden-slot>, <opaque-slot>,
   <read-only-slot>, <self-slot>, <protected-opaque-slot>,
   <protected-hidden-slot>, <protected-read-only-slot>, <scm-slot>,
   <int-slot>, <float-slot>, <double-slot>, <procedure-class>,
   <applicable-struct-class>, <method>, <accessor-method>, <applicable>,
   <applicable-struct>, <generic>, <extended-generic>,
   <generic-with-setter>, <accessor>, <extended-generic-with-setter>,
   <extended-accessor>): Define in Scheme.

  (<boolean>, <char>, <list>, <pair>, <null>, <string>, <symbol>,
   <vector>, <foreign>, <hashtable>, <fluid>, <dynamic-state>, <frame>,
   <vm-continuation>, <bytevector>, <uvec>, <array>, <bitvector>,
   <number>, <complex>, <real>, <integer>, <fraction>, <keyword>,
   <unknown>, <procedure>, <primitive-generic>, <port>, <input-port>,
   <output-port>, <input-output-port>): Define in Scheme.

  (compute-slots): Use build-slots-list helper.

* libguile/goops.h:
* libguile/goops.c (scm_basic_basic_make_class, scm_sys_compute_slots)
  (scm_sys_prep_layout_x): Remove.  These were available to C, but were
  undocumented internals that were dangerous, confusing, and
  unnecessary.

* libguile/goops.c: Add note about variable versus value references.
  Remove internal C routines that were just used during boot, as they
  have been moved to Scheme.
  (scm_basic_make_class): Change to call out to make-standard-class in
  Scheme.
  (scm_sys_make_root_class, scm_sys_bless_applicable_struct_vtable_x)
  (scm_sys_bless_pure_generic_vtable_x, scm_sys_init_layout_x): New
  private helpers.
  (scm_sys_goops_early_init): Change to capture values defined in
  Scheme.