bake: Improve render context and bake record, fix incremental bake bugs. * Used sources and taxonomies are now stored on a per-render-pass basis. This fixes bugs where sources/taxonomies were used for one pass, but that pass is skipped on a later bake because its result is cached. * Bake records are now created for all pages even when they're not baked. Record collapsing is gone except for taxonomy index pages. * Bake records now also have sub-entries in order to store information about each sub-page, since some sub-pages could use sources/taxonomies differently than others, or be missing from the output. This lets PieCrust handle clean/dirty states on a sub-page level.

class Event(object):
    def __init__(self):
        self._handlers = []

    def __iadd__(self, handler):
        self._handlers.append(handler)
        return self

    def __isub__(self, handler):
        self._handlers.remove(handler)
        return self

    def fire(self, *args, **kwargs):
        # Make a copy of the handlers list in case some handler removes
        # itself while executing.
        handlers = list(self._handlers)
        for handler in handlers:
            handler(*args, **kwargs)