Rename `core::comm::oneshot`

[brson]

As in #5300, this function and associated types will be very heavily used once they work reliably. I don't like the name all that much.

[bblum]

perhaps once_pipe for the function name, and OnceChan and OncePort for the type name, to correspond to the OnceClosure traits we're going to have to have.

alternatively, Chan1 and Port1, for increased conciseness but less approachability.

[bblum]

yes hello, this is triage speaking, nothing to contribute since my last comment

[alexcrichton]

Oneshots have been removed.