address: pairs: new helper for JMAP (and maybe lei)

address: pairs: new helper for JMAP (and maybe lei)

Per JMAP RFC 8621 sec 4.1.2.3, we should be able to
denote the lack of a phrase/comment corresponding to an
email address with a JSON "null" (or Perl `undef').

  [
    { "name": "James Smythe", "email": "james@example.com" },
    { "name": null, "email": "jane@example.com" },
    { "name": "John Smith", "email": "john@example.com" }
  ]

The new "pairs" method just returns a 2 dimensional array
and the consumer will fill in the field names if necessary
(or not).

lei(1) may use the two dimensional array as-is for JSON output.