Cope with TAL 1.6 bytecode changes:

* tal:replace now emits something different (optTag)
* constructs with i18n:name emits more complicated bytecode (i18nVariable, etc.)
  to cope with behavioural changes (see http://zope.org/Collectors/Zope3-dev/301)
* probably some more that aren't very interesting (if these any of this is
  interesting at all)