Container-aware Collection::leave_region (#571) #516

Job	Run time
test mdBook	44s
cargo test on ubuntu, rust stable	41s
cargo test on ubuntu, rust 1.78	50s
cargo test on macos, rust stable	59s
cargo test on macos, rust 1.78	1m 11s
cargo test on windows, rust stable	2m 41s
cargo test on windows, rust 1.78	1m 46s
	8m 52s

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