add ::UNDEF and undef->length()

This is a convienience method, designed to make life easier when dealing
with that may or may not be undefined; namely lists.

e.g.

    # ... $warnings *may* be an empty/populated arrayref or undefined
    my $warnings;

    # ...

    # old-school:
    if (!!$warnings && @$warnings) { ... }

     # new-school: (if I may be so bold)
    if ($warnings->length) { ... }

Enabling ->length() to work for any of these contexts stops my eyes from
crossing^W^W^W^W^Wsimplifies the logic required in this common scenario.