Add async constructor & callback support for Dart bindings

[chavic]

Finally taught the Dart bindings how to handle async functions. The runtime now keeps a handle map for Rust futures and foreign (Dart) futures; we generate static factory methods for async constructors, and callback interfaces can actually await their Dart implementations. I also cleaned up the dart_async fixture tests so they call those factories instead of pretending constructors can be async. This should address #66

Summary

• overhaul uniffiRustCallAsync and add shared UniffiForeignFuture runtime glue (handle map, cancellation)
• generate async factories for object constructors and route async free functions through the new helper
• teach callback codegen to emit foreign future structs/typedefs and drive async Dart callbacks safely
• update dart_async fixture/tests to call the async factories (Megaphone.new_(), UdlMegaphone.new_(), etc.) and adjust timing expectations

Testing

cargo test -p dart_async