Introduce and integrate `IntoOwned` trait #306

Job	Run time
test mdBook	49s
cargo test on ubuntu, rust stable	50s
cargo test on ubuntu, rust 1.72	1m 1s
cargo test on macos, rust stable	1m 2s
cargo test on macos, rust 1.72	1m 5s
cargo test on windows, rust stable	2m 18s
cargo test on windows, rust 1.72	1m 52s
	8m 57s

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