* libguile/finalizers.c (async_gc_finalizer):
  (scm_i_register_async_gc_callback): Replace "weak gc callback"
  mechanism with "async gc callback" mechanism.  Very similar but the
  new API is designed to be called a bounded number of times, to avoid
  running afoul of libgc heuristics.
* libguile/weak-list.h: New internal header.
* libguile/Makefile.am (noinst_HEADERS): Add weak-list.h.
* libguile/weak-set.c (vacuum_all_weak_sets):
  (scm_c_make_weak_set, scm_init_weak_set):
* libguile/weak-table.c (vacuum_all_weak_tables):
  (scm_c_make_weak_table, scm_init_weak_table): Arrange to vacuum all
  weak sets from a single async GC callback, and likewise for weak
  tables.

Thanks to Ludovic Courtès for tracking this bug down!