Make a pyproxy of an awaitable python object an awaitable javascript object

[hoodmane]

This PR together with #1158 together finish async/await support. I merged #1158 here even though the change sets are independent because some sort of event loop is necessary in order to test this PR.

This implements await on pyproxy whenever the pyproxy is awaitable. It uses _PyCoro_GetAwaitableIter to check whether the object is awaitable just like in the GET_AWAITABLE opcode. Normally in implementing the await keyword in Python, Python would next use the YIELD_FROM command, but this assumes that Python is yielding into an enclosing coroutine. Instead we use asyncio.ensure_future to schedule the awaitable if necessary and wrap in a future, then we create a promise and connect the promise handles to the future. This could be implemented rather more concisely in Python rather than C but the current bootstrapping scheme makes using Python code to implement PyProxy a bit annoying.

This mirrors the semantics of the underlying python awaitable, so if it is a coroutine then awaiting it multiple times will raise an error, if it is a future-like object then awaiting multiple times is fine.

The error formatting is a bit limited, these PythonError wrappers don't print particularly useful messages in Javascript by default. To improve this requires better javascript / python error integration, something like #1154.

[hoodmane]

@rth @dalcde @phorward Would appreciate review on this one. It'd be nice to get this merged to finish the async/io support. After merging this, I think we should add a section to the usage docs explaining how asyncio works because discoverability is currently poor.

[rth]

Thanks @hoodmane ! A few minor comments below.

Also

• all tests currently rely on eval_code_async, it might be good to add a test that manually generates an awaitable object with run in Python, and passes it to JS.
• could you please test that in JS the then method is defined only if the Python object is awaitable.
• please remove the warning from https://github.com/iodide-project/pyodide/blob/1aea60d91db6dc55cf7108b3f8ddf3c7049af0ff/src/pyodide-py/pyodide/_base.py#L319

[rth]

I think we should re-organize it by submodule as we did for the last release as now it's getting a bit difficult to find things. No action needed in this PR.

[hoodmane]

@rth I think I addressed all the comments in your review.

[rth]

A minor formatting comment otherwise looks good. Thanks!