Support async/await syntax

`async def ` is more explicit and obvious for declaring a coroutine than a normal `def` with yield statements inside it.

`await other_coroutine()` is a bit nicer than `yield from`

Cons:
users might get confused and believe (incorrectly) that this behaves like asyncio stuff, or that they can use stuff like asyncio.sleep()

---

To support async/await, the context helpers need to return values that are both generators and awaitable. Currently they return generators.

A wrapper like this might work:

```python
class AwaitableGenerator:
def __init__(self, generator):
self._g = generator
def __await__(self):
return self._g
def __iter__(self):
return self._g
```