0.9.16.32: weak hash tables

0.9.16.32: weak hash tables
  The implementation is based on cmucl's weak hash table code.

  * scav_vector defers scavenging of weak hash tables until ...

  * ... newspace scavenging at which time the deferred weak hash
    tables are scavenged according to their WEAKNESS type (this
    happens after each scan of newspace)

  * finally just before weak pointers are scanned (i.e. with the
    purpose of breaking them) the weak hash tables are scanned (i.e.
    the appropriate entries are removed) too.