Simplify communication `Message` type #597

Job	Run time
test mdBook	37s
cargo test on ubuntu, rust stable	12s
cargo test on ubuntu, rust 1.78	17s
cargo test on macos, rust stable	23s
cargo test on macos, rust 1.78	28s
cargo test on windows, rust stable	1m 44s
cargo test on windows, rust 1.78	53s
	4m 34s

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