Make `Abelian::negate()` act on borrowed data #339

Job	Run time
test mdBook	58s
cargo test on ubuntu, rust stable	54s
cargo test on ubuntu, rust 1.78	56s
cargo test on macos, rust stable	54s
cargo test on macos, rust 1.78	58s
cargo test on windows, rust stable	1m 42s
cargo test on windows, rust 1.78	1m 42s
	8m 4s

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