* SET-HASH-TABLE/REHASH-THRESHOLD! now preserves the table's usable

* SET-HASH-TABLE/REHASH-THRESHOLD! now preserves the table's usable
  size.  Previously it preserved the physical size.

* SET-HASH-TABLE/REHASH-SIZE! now might reduce the table's usable size
  if the new value adjusts the shrink threshold upwards.

* The REHASH-SIZE of a table is now used to adjust the usable size (as
  documented).  Previously it was adjusting the physical size.

* If an INITIAL-SIZE is given, the table's usable size is initialized
  to that value.  Subsequently, the usable size does not change until
  the table's count exceeds it, at which point normal resizing
  (growing AND shrinking) begins.  If the INITIAL-SIZE is not given or
  is #F, the table is initialized to some unspecified usable size and
  resizes itself according to need.