v1.2.0

• Enable Option<T> types to be transformed (thanks @Desdaemon)
• Support Streams: call function once, "return" multiple times with different data.
• Add FlutterRustBridgeSetupMixin (an optional helper class), which allows custom setup hooks before ffi can be executed.
• Add hint parameter in generated Dart code, allowing users to pass custom data to the Dart executor, thus increasing flexibility.
• Improve panic handling in extreme cases (avoid panic across languages, which is undefined behavior).
• Refactored Handler, now it is much easier to customize your own handler functionality.
• Remove one Box::new(FnOnce), thus enables better inlining for ffi function calls.
• Fix bug: Dart struct(class) is not generated if the struct only appears in the return type #98.
• Add FlutterRustBridgeTimeoutMixin. If used, a timeout exception will be thrown for ffi calls that do not return within time limit.