Support `async` function instrumentation

[appetrosyan]

Hi!. I'm not sure if its possible to test async functions with test-fuzz. I'd like to know if it is possible, and if it is, to please add the instructions to do so into documentation.

[smoelius]

Hi, @appetrosyan. To be honest, I'm not sure whether test-fuzz works with async functions. I agree it would be nice, though. It's something I will have to investigate.

[appetrosyan]

I thought that it probably didn’t, but wanted to make sure.

I would imagine that a sync fuzzing would be very useful for most web frameworks. I even have a few ideas for how to do it incrementally: first by running the asynchronous functions synchronously, to work out the types and macros, but also to provide a way of testing asynchronous functions of some kinds.

Then, perhaps, to try and test asynchronous functions with a patched runtime, where the timing and scheduling is also one of the variables.