remove unsound experimental coroutines

[cyrgani]

The following problematic piece of code in Handle::lens is always UB:

let mut base = std::mem::MaybeUninit::<T>::uninit();
let field = f(std::mem::transmute(base.as_mut_ptr())) as *mut _ as *mut u8;

The MaybeUninit::as_mut_ptr documentation says:

Reading from this pointer or turning it into a reference is undefined behavior unless the MaybeUninit<T> is initialized.

But f takes a mutable reference as argument, so this is UB.

Since I did not see an easy fix for this and it is only an experimental API, it should be fine to just remove it.

Closes #723.

[cyrgani]

Closing this since #798 includes this change.