dillon1000/react

[rust] Codegen AST visitor

Updates `estree` codegen to emit a Visitor trait (temporarily named `Visitor2` 
since there is a hand-rolled one that some code is using). We use knowledge of 
the grammar to only visit fields whose type is a Node or Enum, or an "object" 
type that opts into being visitable. The latter is used for Function and Class. 

This will make it much easier to write the semantic analyzer.