Proposal: add Channel.receive_first_with_index and Channel.send_first_index

[joatca]

Channel.receive_first and Channel.send_first discard the index returned by the underlying Channel.select. To handle cases where the channel that performed an operation is significant, I propose Channel.receive_first_with_index for receiving and either a new method Channel.send_first_index or returning the index directly from Channel.send_first and allowing using code to discard it. The discussion at crystal-lang/crystal-book#279 (review) suggests that Channel.select is intended as a low-level interface and that the *_first methods are considered the high-level interface.

Implementation of this should be trivial so I'm willing to submit a PR if everyone is agreeable.

[straight-shoota]

Duplicate of #10411