allow overloading fallback (specifically to get cmp)

[rt.cpan.org #42573]

I'm not sure that falling back to stringification will be the best
behavior, although it will be better than nothing.  Given the
following three stringifications:

  albert@example.com
  "Betty O'Connor" <betty@example.com>
  charlie@example.com

...I think sorting by stringification will be weird.
document the only-meant-for-list behavior of parse

[rt.cpan.org #39299]
dzilify
avoid warnings now that we use warnings
v1.899

          repackaged, pointed at new bugtracker

          fallback overloads are enabled, so "cmp" will now compare with string
          overloads... which still might not be what you expect

          code has been futzed with to rely on some 5.6-isms, rather than
          trying to be 5.005 compatible

          a warning has been added: calling ->parse in scalar context gets
          undefined behavior