Fixed the weak cell implementation (for weak hash tables).

Fixed the weak cell implementation (for weak hash tables).

* libguile/hashtab.c (wcar_cell_descr): New.
  (wcdr_cell_descr): New.
  (scm_weak_car_cell): Use `GC_malloc_explicitly_typed ()' instead of
  `scm_gc_malloc_pointerless ()'.
  (scm_weak_cdr_cell): Likewise.
  (SCM_WEAK_CELL_WORD_DELETED_P): Use `SCM_CELL_OBJECT' instead of
  `SCM_CELL_WORD'.
  (scm_hash_fn_get_handle): Call `GC_disable ()' before calling
  `scm_fixup_weak_alist ()' and `GC_enable ()' afterwards.
  (scm_hash_fn_create_handle_x): Likewise.
  (scm_hash_fn_remove_x): Likewise.
  (scm_hashtab_prehistory): Initialize WCAR_CELL_DESCR and
  WCDR_CELL_DESCR.

git-archimport-id: lcourtes@laas.fr--2005-libre/guile-core--boehm-gc--1.9--patch-8