Simplify cancellation logic

[talex5]

Before, the fiber context held an atomic recording whether the operation had succeeded or been cancelled, to ensure that only one of these things happened. However, this doesn't need to be atomic for single-threaded operations, and even for parallel operations, we might perfer to keep the atomic state elsewhere.

Now, operations just set a regular function and it's up to the operation to ensure it's atomic if needed.

This is needed to implement lock-free waiters. Otherwise we have this problem:

1. Domain 1 starts waiting for something from another domain.
2. Domain 2 prepares to provide a value to it.
3. Domain 1 cancels the operation and continues.
4. Domain 1 starts a new operation with a new cancel function.
5. Domain 2 sees the operation isn't cancelled and sends the value.

This isn't a problem when using a lock because both domains take the lock. Currently, the waiters module still uses a lock and it now provides its own mutex that works the same as before.

Some other related improvements:

• In eio_linux, we no longer wait for the cancel response before continuing.

  Before, cancelling an io_uring request would wait for the result of the cancellation before the cancel operation returned. But this isn't very useful, makes cancelling lots of things very slow since you can only do one at a time, and doesn't work with the new cancellation system where you can't switch fibers while cancelling.

• The "Cancelling multiple jobs" test has been removed. It was testing what happens when a cancel function switches fibers, but this is no longer allowed.

• In eio_luv, we only report cancellation if the operation didn't succeed.

  This changes the signature of Low_level.await_with_cancel slightly, since it now needs to know whether the operation was successful or not.

• Fixed a bug in the mock domain manager used for the README examples. I think this was wrong before too, but it was only detected with the new system.

TODO:

• Run benchmarks to check this didn't break anything