asyncio bridge improvements

[Fuyukai]

This adds some improvements to the asyncio bridge, such as:

• A decorator that allows easier awaiting of foreign coroutines
• The ability to pass keyword arguments to a corofunc
• The ability to pass a coroutine object straight into the loop.

The first one was my main use case for one of my projects; it's easier than running the loop manually every time.

[dabeaz]

I'm going to merge this, but some changes will be made to the *args, *kwargs calling convention. Throughout Curio, there has been effort to standardize the calling convention of functions that run other functions (e.g., `spawn(func, args),run_in_thread(func, *args), etc.). Positional arguments are passed to the function itself whereas keyword arguments are used for configuration and options to the operation itself. To pass keyword arguments, you would usepartial()`.