Demonstrate `BytesSlab` with abstracted bytes storage #737

Job	Run time
test mdBook	49s
cargo test on ubuntu, rust stable	17s
cargo test on ubuntu, rust 1.78	20s
cargo test on macos, rust stable	42s
cargo test on macos, rust 1.78	28s
cargo test on windows, rust stable	50s
cargo test on windows, rust 1.78	49s
	4m 15s

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