implement features required for structured concurrency

[graingert]

I'd like to be able to implement a timeout or TaskGroup context manager like in Trio/asyncio:

import sleep from 'effection';

function* example () {
    using* tg = new TaskGroup();
    tg.startSoon(sleep, 10);
    yield* sleep(1);
}

This requires access to the error:
#49

and support for generator functions in disposing, specifically a Symbol.generatorDispose that supports a generator function that gets called when using using*