ci: fail on clippy warnings #236

Job	Run time
test (stable)	26s
test (beta)	22s
clippy	24s
doc	10s
examples (stable)	21s
examples (beta)	19s
test (nightly-2023-04-01)	24s
formatting	1m 4s
Build for a no_std target (stable)	16s
examples (nightly-2023-04-01)	22s
Build for a no_std target (beta)	20s
udeps	4m 33s
build (stable)	14s
Build for a no_std target (nightly-2023-04-01)	21s
build (beta)	20s
coverage	2m 24s
build (nightly-2023-04-01)	23s
deny	4m 18s
Rust 1.56.0 MSRV	2m 8s
Rust 1.57.0 MSRV	2m 8s
	21m 17s

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