allow coroutines with Python 3 syntax as async def functions about circuits

[ri0t]

Some motivation: https://www.reddit.com/r/Python/comments/q9326p/python_stands_to_lose_its_gil_and_gain_a_lot_of/ 🎆 🍾

[spaceone]

you mean asyncio ?

[spaceone]

The starting point for async functions is:
https://github.com/circuits/circuits/compare/async?expand=1

See #281 (comment)

[spaceone]

Relevant documentation is here:

https://docs.python.org/3.7/glossary.html

https://docs.python.org/3.7/reference/datamodel.html#coroutines

https://docs.python.org/3.7/reference/compound_stmts.html#coroutines

We have to make some design decisions.
Do we want to support backwards compatibility with yield?
Should every @handler be a async coroutine? Or are functions still allowed?
Can we drop the CallValue() wrapper class and use the return value of a async function?
Sleep has to implement await()