feat: oneshot channel

[tisonkun]

This is derived from https://github.com/faern/oneshot/.

Special thanks to @faern and @Cassy343 for the original work. I made my own for mea, since mea needs not support synchronized receiving functions. And this can simplify the code a lot.

1. Receiver::drop no longer needs to handle RECEIVING or AWAKING state. Because only try_recv avaialble and any RECEIVING state would only happen on the Recv future.
2. Reuse the same busy loop logic for Recv when handling AWAKING state in a poll.

This also make use of the patch faern/oneshot#61 and I suppose that it provides a better impl for implementing Recv as a future, rather than under the name AsyncReceiver and thus no async_channel.

Other refs -

• Relax atomic orderings + fix parking/unparking UB faern/oneshot#17
• Fix and improve the Future impl on Sender faern/oneshot#20
• Fix try_recv (And add a bunch of loom based tests) faern/oneshot#23
• Handle UNPARKING state in Receiver::drop() faern/oneshot#60

[tisonkun]

Already invariant due to UnsafeCell<T>.