Update to 4.0

Upstream changes:
4.0  Fri Nov 16 00:06:54 CET 2018
        - 4.0 pre-release, do not use other than for testing.
        - SECURITY IMPLICATION: this release enables allow_nonref by default
          for compatibnility with RFC 7159 and newer. See "old" vs. "new"
          JSON under SECURITY CONSIDERATIONS.
        - reworked the "old" vs. "new" JSON section.
        - add ->boolean_values to provide the values to which booleans
          decode (requested by Aristotle Pagaltzis).
        - decode would wrongly accept ASCII NUL characters instead of
          reporting them as trailing garbage.
        - work around what smells like a perl bug w.r.t. exceptions
          thrown in callbacks.
        - incremental parser now more or less respects allow_nonref.
        - json_xs json-pretty now enables canonical mode.
        - add documentation section about I-JSON.
        - minor documentation fixes/updates.


(wen)