+ Add comma token
+ Add tuple operator
+ Add tuple value type
+ Implement display for `tree::Node` and all related types
+ Add support for right-to-left chaining of operators with same precedence
The priority of chaining of operators with the same precedence remains left-to-right.
Only if two consecutive operators with the same precedence are right-to-left, they will be ordered right-to-left.
Implements #4.