sync: oneshot::Receiver::is_closed()

[cratelyn]

this commit introduces a new method to
tokio::sync::oneshot::Receiver<T>.

this method returns true if the channel is closed. this is similar to the existing
tokio::sync::mpsc::UnboundedReceiver::is_closed() and tokio::sync::mpsc::Receiver::is_closed() methods.

see:

• https://docs.rs/tokio/latest/tokio/sync/mpsc/struct.Receiver.html#method.is_closed
• https://docs.rs/tokio/latest/tokio/sync/mpsc/struct.UnboundedReceiver.html#method.is_closed
• sync: oneshot::Receiver::{is_empty, is_closed} #7137 (comment)

[cratelyn]

there are still some open questions about the semantics of this method. see in particular, this thread of discussion from #7137.

[cratelyn]

i see issues in ci at the moment, but i am marking this as ready for review because i believe these errors to be unrelated.

⚠️ Failed to start an instance: INVALID_ARGUMENT: Not Found 404 Not Found
POST [https://compute.googleapis.com:443/compute/v1/projects/cirrus-ci-community/zones/us-central1-c/instances](https://compute.googleapis.com/compute/v1/projects/cirrus-ci-community/zones/us-central1-c/instances)