Make callback optional for group_by() and index_by().
Use dict collections' values for max_(), min_(), shuffle(), sort_by(), and to_list().
Pass kargs to invoked method.
Remove unneeded early returns for min_() and max_().
Remove unneeded handling of None callback as _iter_callback() will take care of it.
Make callback optional for sort_by().
Add additional test cases for collections which have special handling for dicts.
Update docstrings with Args/Returns sections.