Use of removed boost::coroutine2::detail::forced_unwind struct

[juanalday]

Since boost 1.74, boost::coroutine2::detail::forced_unwind no longer exists . Since this is already mentioned in quantum itself this is just an omission.

It is also mentioned as an issue in https://github.com/boostorg/coroutine2/issues/23

To Reproduce Just build with boost >=1.74.