Moved the lexer and parser instances in separate lexer/parser singletons

This allows reusing the filter lexer/parser in tag parsing without
rebuiling the lexer/parser every time.
Django's `cached_property` won't work with what I need in the future, I think. By doing it the way I did we can simply reset `_fl` or `_sl` to None to force a rebuilding of a lexer/parser if at a later time e.g. productions have been added.