Support async functions

[wessberg]

This PR fixes #67 .

It simply adds checks for async functions where functions are expected.

[addaleax]

@wessberg Thanks, but I don’t think this quite works yet – asyncfunction is not the result of typeof, it’s coming from the result of Object.prototype.toString.call(async function() {}).

(Also, feel free to omit the dist/ changes in PRs – they are overwritten anyway and don’t interact well with git.)

I’ll try to get this fixed soon, but the approach would probably require using something like the _function entry. Also, I think at this point both generator functions and async generator functions are unsupported as well, and would likely need to be addressed in a similar manner as normal async functions.

[addaleax]

Closing as there hasn’t been any reply in a long time