Fix SRFI 69 HASH-TABLE-FOLD.

Fix SRFI 69 HASH-TABLE-FOLD.

The specification makes no mention of any constraint that the combine
procedure not update the hash table, so we must be prepared for that
case.  The old definition for HASH-TABLE-FOLD now has the new name
%HASH-TABLE-FOLD, used internally by the procedures that need to fold
but need not update the hash table while doing so.