dillon1000/react

[rust] ESTree representation with basic serialization support

Start of an `estree` crate for representing ESTree with serialization to/from 
ESTree JSON. To get the serialization to work quickly I took some shortcuts and 
uses a slightly less precise modeling. Longer-term we can write some custom 
serialization code and get more precise types (eg avoid the `ExpressionLike` 
enum in favor of proper Expression and ExpressionOrFoo types).