Native web fixes.

[TannerRogalsky]

iced_futures

• Since there are several "platform-specific" type aliases (and a new function), I moved them into their own mod. Just makes it easier to read and minimizes the number of #[cfg(...)] annotations.
• Added a new function boxed_stream that takes a stream of the type supported by the target architecture and boxes it accordingly.

iced_native

• Use the iced_futures types and functions to allow the iced_native to compile on the wasm32-unknown-unknown target.

[hecrj]

Awesome! The module approach is much cleaner.

Thank you 🙇