skiplist: a new iteator which is sendable among threads

[SpadeA-Tang]

Ref #1090

This PR implements a wrapper of the skiplist which supports creating iteator that is sendable among threads.
More tests or comment can be added if this is needed.

[taiki-e]

I think we could add a method that takes self: Arc<Self> instead of creating a new type. Like:

• https://docs.rs/tokio/latest/tokio/sync/struct.Mutex.html#method.lock_owned
• https://docs.rs/async-lock/latest/async_lock/struct.Mutex.html#method.lock_arc

[SpadeA-Tang]

Thanks for the reply. It's a bit late here, I will see it tomorrow.

[SpadeA-Tang]

I have modified the code. PTAL again, thanks @taiki-e . If this is desired, I will add more tests.