Clean: Actually use HvUSEDKEYS() instead of HvKEYS()

Clean: Actually use HvUSEDKEYS() instead of HvKEYS()

This:

  commit 8aacddc1ea3837f8f1a911d90c644451fc7cfc86
  Author: Nick Ing-Simmons <nik@tiuk.ti.com>
  Date:   Tue Dec 18 15:55:22 2001 +0000

      Tidied version of Jeffrey Friedl's <jfriedl@yahoo.com> restricted hashes
       - added delete of READONLY value inhibit & test for same
       - re-tabbed

      p4raw-id: //depot/perlio@13760

essentially deprecated HvKEYS() in favor of HvUSEDKEYS(); this is
explained in line 144 (now 313) of file `hv.h':

  /*
   * HvKEYS gets the number of keys that actually exist(), and is provided
   * for backwards compatibility with old XS code. The core uses HvUSEDKEYS
   * (keys, excluding placeholdes) and HvTOTALKEYS (including placeholders)
   */

This commit simply puts that into practice, and is equivalent to running
the following (at least with a35ef416833511da752c4b5b836b7a8915712aab
checked out):

  git grep -l HvKEYS | sed /hv.h/d | xargs sed -i s/HvKEYS/HvUSEDKEYS/

Notice that HvKEYS is currently just an alias for HvUSEDKEYS:

  $ git show a35ef416833511da752c4b5b836b7a8915712aab:hv.h | sed -n 318p
  #define HvKEYS(hv)		HvUSEDKEYS(hv)

According to `make tests':

  All tests successful.