Implement a free `sorted_unstable` #581

Job	Run time
test	1m 30s
msrv	1m 27s
check	22s
check format	6s
check (--no-default-features)	29s
check (--no-default-features --features use_alloc)	7s
check (--all-targets --all-features)	31s
All checks succeeded	3s
	4m 35s

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