Implement Clone for AbortHandle

[aschweig]

This is a small change so that AbortHandle implements Clone.

Motivation

The the module level docs indicate

Each task can only have one JoinHandle, but it can have more than one AbortHandle.

Cloning is convenient for AbortHandles given there can be more than one.

Solution

The proposed solution is similar to JoinHandle's abort_handle method:

impl Clone for AbortHandle {
    /// Returns a cloned `AbortHandle` that can be used to remotely abort this task.
    fn clone(&self) -> Self {
        self.raw.ref_inc();
        Self::new(self.raw)
    }
}