RHH: Use `usize` and `u64` better #281

Job	Run time
test mdBook	52s
cargo test on ubuntu, rust stable	59s
cargo test on ubuntu, rust 1.72	1m 5s
cargo test on macos, rust stable	1m 16s
cargo test on macos, rust 1.72	57s
cargo test on windows, rust stable	2m 6s
cargo test on windows, rust 1.72	2m 11s
	9m 26s

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