specific addition of marshalling None to <boolean>0</boolean>.

Benefits gained:

  - Uses Expat when available (like in Zope), which makes unmarshalling
    (receiving XML-RPC calls) much,much faster.

  - New library imports other python modules lazily, which helps speed up
    Zope startup time.

  - Faster escape method for outgoing calls.

  - Fixes for doubles and larger integers.

  - Allow XML-RPC calls without an empty <params /> (fixes Zope Collector
    #465)

Additional fix:

  - Fix SlowParser to handle CDATA sections as well. With the ExpatParser
    this is less of a problem (fixes Zope Collector #547). Also see Python
    SF issue #601534.