Reimplement %allocate-instance in Scheme

Reimplement %allocate-instance in Scheme

* libguile/goops.c (scm_sys_clear_fields_x): New function.
  (scm_sys_allocate_instance): Remove.  It was available to C but not to
  Scheme and it's really internal.
* libguile/goops.h: Remove scm_sys_allocate_instance.

* module/oop/goops.scm (%allocate-instance): Implement in Scheme, using
  allocate-struct and %clear-fields!.
  (make, shallow-clone, deep-clone, allocate-instance): Adapt to
  %allocate-instance not taking an initargs argument.