Add cross-compatibility functionality between backends

It would be really awesome to be able to run your app on trio while still retaining the ability to spawn asyncio tasks or calling asyncio coroutines. I'm not sure how far trio's guest mode will take us, but this is definitely worth exploring. The trio-asyncio library should give us some clues on how to accomplish this.

Potential features we would like to have:

- A way to spawn asyncio tasks from any backend
- A way to call asyncio coroutines from any backend
- A way to use synchronization primitives across backends