* AST::Visitor -> AST::NodeVisitor
* Scope & ScopeVisitor
* Useless assignment rule
* Instance vars and useless assignments
* Multiple assigns one by one
* Support outer scope
* Variable used in the useless assignment
* Support OpAssign & MultiAssign
* Captured by block
* Variable, Assignment, Reference & Refactoring
* Variable has references, Assignment can be referenced
* Branch entity
* Handle useless assignments in branches
* Handle assignments in a loop
* Handle branch equality
* Handle special var `$?` assignment
* Improve captured by block stuff
* Avoid assignments in property definitions
(UselessAssign rule reports an warning)
* Support MacroIf and MacroFor branches
* Handle assignments with shadowed vars in inner scopes
* Add method arguments as scope variables
* Handle case if branch is blank
* Top level scope
* Handle case when branch is nop?
