WIP - make AVs use NULL for empty slots

This is an experiment.  I also intend to try &PL_sv_placeholder and
then see which is better.

Remaining test failures:

Test Summary Report
-------------------
../ext/B/t/showlex.t                                            (Wstat: 0 Tests: 15 Failed: 1)
  Failed test:  1
../dist/Storable/t/freeze.t                                     (Wstat: 256 Tests: 21 Failed: 1)
  Failed test:  21
  Non-zero exit status: 1
../dist/ExtUtils-ParseXS/t/002-more.t                           (Wstat: 11 Tests: 1 Failed: 0)
  Non-zero wait status: 11
  Parse errors: Bad plan.  You planned 28 tests but ran 1.
../cpan/ExtUtils-Constant/t/Constant.t                          (Wstat: 0 Tests: 33 Failed: 1)
../ext/Devel-Peek/t/Peek.t                                      (Wstat: 11 Tests: 89 Failed: 1)
  Failed test:  17
  Non-zero wait status: 11
  Parse errors: No plan found in TAP output
../ext/IPC-Open3/t/IPC-Open3.t                                  (Wstat: 512 Tests: 27 Failed: 2)
  Failed tests:  26-27
  Non-zero exit status: 2
  Parse errors: Bad plan.  You planned 37 tests but ran 27.