Allow `Q: ?Sized` in `Itertools::contains` #985

Job	Run time
doc	8s
check format	7s
check	8s
miri	2m 47s
check (--no-default-features)	7s
msrv	40s
semver-checks	2m 21s
check (--no-default-features --features use_alloc)	8s
check (--all-targets --all-features)	17s
test	56s
All checks succeeded	1s
	7m 40s

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