Fix Clippy warnings and enable Clippy in CI. #2731

Job	Run time
softhsm2-test (1.70.0, hsm,hsm-tests-pkcs11)	5m 24s
pykmip-test (1.70.0, hsm)	5m 48s
build (ubuntu-latest, 1.70.0, --no-default-features)	7m 14s
softhsm2-test (stable, hsm,hsm-tests-pkcs11)	4m 57s
pykmip-test (1.70.0, hsm,hsm-tests-kmip)	6m 4s
build (ubuntu-latest, 1.70.0)	9m 12s
softhsm2-test (beta, hsm,hsm-tests-pkcs11)	5m 2s
pykmip-test (stable, hsm)	5m 13s
build (ubuntu-latest, 1.70.0, --features all)	8m 34s
pykmip-test (stable, hsm,hsm-tests-kmip)	5m 28s
build (ubuntu-latest, stable, --no-default-features)	6m 57s
pykmip-test (beta, hsm)	5m 10s
build (ubuntu-latest, stable)	8m 10s
pykmip-test (beta, hsm,hsm-tests-kmip)	5m 50s
build (ubuntu-latest, stable, --features all)	8m 52s
build (ubuntu-latest, beta, --no-default-features)	6m 30s
build (ubuntu-latest, beta)	7m 55s
build (ubuntu-latest, beta, --features all)	7m 56s
	2h 0m 16s

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