remove weak pairs, rewrite weak vectors

* libguile/weak-vector.c:
* libguile/weak-vector.h: Renamed from weaks.[ch].  Remove weak pairs.
  They were not safe to access with `car' and `cdr'.  Remove weak alist
  vectors, as we have weak tables and sets.  Reimplement weak vectors,
  moving the implementation here.

* libguile/vectors.c:
* libguile/vectors.h: Remove the extra header word.  Use
  scm_c_weak_vector_ref / scm_c_weak_vector_set_x to access weak
  vectors.

* libguile/snarf.h: Remove the extra header word in vectors.

* libguile/threads.c (do_thread_exit, fat_mutex_lock, fat_mutex_unlock):
  Instead of weak pairs, store thread-owned mutexes in a list of
  one-element weak vectors.

* libguile/guardians.c (finalize_guarded): Similarly, store object
  guardians in a list of one-element weak vectors.

* libguile/modules.c (scm_module_reverse_lookup): We no longer need to
  handle the case of weak references.

* libguile/print.c (iprin1): Use the standard vector accessor to print
  vectors.

* libguile.h:
* libguile/Makefile.am:
* libguile/gc-malloc.c:
* libguile/gc.c:
* libguile/goops.c:
* libguile/init.c:
* libguile/objprop.c:
* libguile/struct.c: Update includes.

* module/ice-9/weak-vector.scm: Load weak vector definitions using an
  extension instead of %init-weaks-builtins.

* test-suite/tests/weaks.test: Use the make-...-hash-table names instead
  of the old alist vector names.

libguile.h

@@ -117,7 +117,7 @@ extern "C" {
 #include "libguile/vports.h"
 #include "libguile/weak-set.h"
 #include "libguile/weak-table.h"
-#include "libguile/weaks.h"
+#include "libguile/weak-vector.h"
 #include "libguile/backtrace.h"
 #include "libguile/debug.h"
 #include "libguile/stacks.h"