Do not show hidden attributes in autocompletion.
Normalize operator priorities.
Remove meaningless methods of `Node`.
Slightly improve ImportError messages.
Restore BUNDLE_DIR.
Refine `Parser.infix`.
Allow to use trailing variadic operators.

If `R` is variadic, `a R b R c R` == `a R b R c`. For example:

    1, 2, 3,
Implement `yield from`.