ci: fail on clippy warnings #237

Job	Run time
examples (stable)	20s
examples (beta)	20s
examples (nightly-2023-04-01)	22s
formatting	1m 7s
doc	31s
build (stable)	13s
clippy	23s
build (beta)	22s
Rust 1.56.0 MSRV	2m 9s
build (nightly-2023-04-01)	23s
Build for a no_std target (stable)	12s
Rust 1.57.0 MSRV	2m 7s
test (stable)	24s
Build for a no_std target (beta)	21s
test (beta)	21s
Build for a no_std target (nightly-2023-04-01)	20s
test (nightly-2023-04-01)	29s
coverage	2m 30s
deny	4m 21s
udeps	4m 24s
	21m 39s

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