bpo-47062: Implement asyncio.Runner context manager

[asvetlov]

https://bugs.python.org/issue47062

[zware]

I have a few mostly grammatical suggestions, but otherwise LGTM.

There may be a few comments that no longer apply; I started reviewing before you finished pushing changes and now I can't find them to remove them :)

[asvetlov]

Thanks, @zware
Notes are fixed

[asvetlov]

I've modified the Runner class to use lazy initialization: __enter__ or the first run() creates embedded loop and contextvars.Context.

@1st1 does it look better for you?

[asvetlov]

Explicit runner.get_context() method is dropped.
An embedded context is still created at the time of embedded loop initialization.

[1st1]

Post merge: can you rename "factory" to "loop_factory"?