dillon1000/react

[rust] allow complex types in estree codegen

Uses the `syn` crate, which can parse various Rust syntax forms, to parse the 
`type` field from json schema description. This allows us to describe complex 
types like `"type": "Vec<Option<ArrayElement>>"` directly, rather than requiring 
flags like nullable, plural, and nullable_item. The main flag that i'm keeping 
is "optional", which is used to indicate when the field itself (not the value) 
is optional.