Optional argument to resume_execution allows value stack underflow. about wasmi

[eira-fransham]

Neither of the suggested fixes work, I'm afraid. It's not an optional argument, really, it just has type Option. None doesn't mean that the user forgot to supply a value or didn't care, it means that the resumption should return void. I mentioned getting rid of it in my review of your PR, but I was wrong - the return_val is the return value of the yield, not the return type of the yield (which is what I thought it was). The fix for this is to have resume_execution check the (runtime) type of return_val against the expected type.