dillon1000/react

Add HIR "tree" visitor and use it for codegen, alternate printer

Refactors Codegen to extract the core "visit IR as a tree" logic separately from 
the code to emit JS: 

* `HIRTreeVisitor` is a new helper that visits the HIR as a tree. You call 
`visitTree(ir, yourVisitor)` and it drives visiting of the IR, tracking blocks 
and scopes and calling methods as appropriate. 

* `Codegen` is now implemented as a Visitor implementation. For example 
`enterBlock()` creates an empty `Array<t.Statement>`, `leaveBlock()` wraps that 
in a `t.BlockStatement`, etc. 

* `printHIRTree()` is a new IR printer (implemented as a visitor) that prints 
the HIR in tree form, so it retains the original shape of the code but with each 
block replaced with its IR equivalent.