bother; this fixes the loopDeps optimization, that reduced the size of
the minified output file; the callback was being called in all cases,
not just when there were dependencies.
Pass the @options.bindContext into functions that are evaluated during
Binding::set.
Add a rivets_index data value that mirrors ${object_name}_index; this
allows for general-purpose functions to always look at the former.  It
makes data-class-even="workers:isEven" possible.