[rust-compiler] Eliminate double AST serialization with RawValue

Change CompileResult.ast from Option<serde_json::Value> to
Option<Box<RawValue>>, serializing the AST directly to a JSON string
in compile_program rather than going through an intermediate Value.
This avoids a full extra serialization pass (File→Value→String becomes
File→String) when the NAPI layer serializes the final result.