reimplement hashtab.c's weak hash tables in terms of weak-table.c

reimplement hashtab.c's weak hash tables in terms of weak-table.c

* libguile/hashtab.c:
* libguile/hashtab.h: Reimplement the weak hash table implementation in
  terms of weak tables.  All is well except for the horrific hack for
  hashx tables.

* libguile/weak-table.h:
* libguile/weak-table.c (scm_make_weak_key_hash_table)
  (scm_make_weak_value_hash_table, scm_make_doubly_weak_hash_table)
  (scm_weak_key_hash_table_p, scm_weak_value_hash_table_p)
  (scm_doubly_weak_hash_table_p): Move these definitions here.