Case 0 and 1 of `k_smallest` and variants #850

Job	Run time
check format	14s
msrv	42s
check	20s
doc	13s
check (--no-default-features)	21s
semver-checks	2m 25s
test	53s
check (--no-default-features --features use_alloc)	12s
check (--all-targets --all-features)	25s
All checks succeeded	1s
	5m 46s

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