via <%page> or default_filters (but not those within the filter)
- added buffer_filters argument, defines filters applied to the return value
of buffered/cached/filtered %defs, after all filters defined with the %def
itself have been applied.  allows the creation of default expression filters
that let the output of return-valued %defs "opt out" of that filtering
via passing special attributes or objects.
- added support for "class" structures in ast parsing (i.e. class-level
data members wont get added to the "declared" list)