Synchronous in Dart

If you need to generate synchronous functions in Dart, you can use SyncReturn<T> as the return type.

We suggest only do this for very quick Rust functions, or the Dart UI will be blocked.

Currently, SyncReturn supports String, Vec<u8> and primitive types(bool, u8, u16, u32, u64, i8, i16, i32, i64, f32, f64). Please open an issue if you need other types.