Allow for async functions to be used almost everywhere

[kraih]

This is an alternative to #1452. It has the advantage that async functions work pretty much everywhere automatically. It does have the downside that it deactivates the Future::AsyncAwait feature for detecting unhandled promises however. So it becomes much easier to miss exceptions in async functions.

[kraih]

If we decide to go with this solution, we should probably switch to a much more efficient internal alternative to finally (which requires the creation of yet another promise currently).

[kraih]

This solution is almost there, we just need a way to detect and throw unhandled rejections during DESTROY.