parsers::parse() - New function, to replace property_bag::parse_*()
parsers::parse() - New function, to replace property_bag::parse_*()

property_bag::parse_or_none() would return an Option.  We will no
longer need it because we'll iterate over attributes that *are*
present, not try to match against attributes that may not be there.

property_bag::parse_or_default() will not be needed, since the node
creation functions will set up default values themselves.

property_bag::parse_or_value() will not be needed for the same reason.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg