Fix clippy warnings #543

Job	Run time
Book tests	1m 47s
Bitrot check	1m 32s
Code coverage	3h 2m 16s
Build target wasm32-unknown-unknown	2m 13s
Intra-doc links	1m 52s
Build target wasm32-wasi	1m 57s
Rustfmt	16s
Test on ubuntu-latest	11m 4s
Test on windows-latest	14m 22s
Test on i686-unknown-linux-gnu	22m 45s
Test on macOS-latest	8m 57s
Test on i686-unknown-linux-gnu with beta features	22m 56s
Test on i686-unknown-linux-gnu with nightly features	22m 33s
Test on ubuntu-latest with beta features	10m 53s
Test on windows-latest with beta features	14m 34s
Test on macOS-latest with beta features	9m 13s
Test on ubuntu-latest with nightly features	11m 12s
Test on windows-latest with nightly features	14m 56s
Test on macOS-latest with nightly features	8m 54s
	6h 4m 12s

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