halo2_proofs: Fix clippy lints #4107

Job	Run time
Test on i686-unknown-linux-gnu with beta features	15m 20s
Test on i686-unknown-linux-gnu with nightly features	17m 11s
Build target wasm32-unknown-unknown	3m 45s
Test on i686-unknown-linux-gnu	15m 22s
Test on windows-latest	12m 28s
Bitrot check	46s
Build target wasm32-wasi	3m 32s
Test on ubuntu-latest	9m 27s
Test on windows-latest with beta features	12m 0s
Code coverage	3m 56s
Rustfmt	11s
Intra-doc links	3m 29s
Test on windows-latest with nightly features	13m 18s
Book tests	3m 16s
Test on macOS-latest with nightly features	8m 19s
Test on ubuntu-latest with beta features	9m 9s
Test on ubuntu-latest with nightly features	10m 27s
Test on macOS-latest	8m 54s
Test on macOS-latest with beta features	8m 30s
	2h 39m 20s

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