disallow get-handle / create-handle! of weak hash tables

disallow get-handle / create-handle! of weak hash tables

* libguile/hashtab.c (scm_hashq_get_handle, scm_hashq_create_handle_x)
  (scm_hashv_get_handle, scm_hashv_create_handle_x)
  (scm_hash_get_handle, scm_hash_create_handle_x)
  (scm_hashx_get_handle, scm_hashx_create_handle_x): Don't allow these
  functions to be called on weak hash tables, as we have no idea when
  the GC will null out fields of the handle, and set-cdr! won't register
  disappearing links, and set-car! would never work of course.