Add more foreign object interfaces

Add more foreign object interfaces

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_make_foreign_object_1)
  (scm_make_foreign_object_2, scm_make_foreign_object_3)
  (scm_make_foreign_object_n): Change to take void * arguments, and to
  add a comment to the header indicating that these are convenience
  constructors.

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_foreign_object_unsigned_ref)
  (scm_foreign_object_unsigned_set_x): New functions, equivalent to the
  old scm_foreign_object_ref and scm_foreign_object_set_x.

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_foreign_object_signed_ref)
  (scm_foreign_object_signed_set_x): New functions taking
  scm_t_signed_bits.

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_foreign_object_ref)
  (scm_foreign_object_set_x): New functions that take void*.