Add weak-vector-length, weak-vector-ref, weak-vector-set!

Add weak-vector-length, weak-vector-ref, weak-vector-set!

* libguile/weaks.c (scm_is_weak_vector, scm_c_weak_vector_length):
  (scm_c_weak_vector_ref, scm_c_weak_vector_set_x): New interfaces for
  dealing with weak vectors from C.
  (scm_weak_vector_length, scm_weak_vector_ref, scm_weak_vector_set_x):
  New Scheme interfaces to weak vectors; to be used instead of
  vector-length, vector-ref, etc.

* module/ice-9/weak-vector.scm: Export the new interfaces.

* doc/ref/api-memory.texi (Weak vectors): Document them.