Stabilize mpmc_channel

[obeis]

tracking issue: #126840

[rustbot]

r? @jhpratt

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[dtolnay]

SendTimeoutError::Disconnected renders as "sending on a disconnected channel", which is different from SendError and TrySendError::Disconnected (all of which are the same failure mode). Let's pick one of these messages and use it for all three types.

rust/library/std/src/sync/mpsc.rs

Lines 1168 to 1172 in e19d321

	impl<T> fmt::Display for SendError<T> {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	"sending on a closed channel".fmt(f)
	}
	}

rust/library/std/src/sync/mpsc.rs

Lines 1190 to 1197 in e19d321

	impl<T> fmt::Display for TrySendError<T> {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	match *self {
	TrySendError::Full(..) => "sending on a full channel".fmt(f),
	TrySendError::Disconnected(..) => "sending on a closed channel".fmt(f),
	}
	}
	}

(Can be a separate PR or in this PR.)

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