`Positions` based on `Enumerate` #626

Job	Run time
msrv	40s
check	12s
test	58s
check format	11s
check (--no-default-features)	14s
check (--no-default-features --features use_alloc)	14s
check (--all-targets --all-features)	26s
All checks succeeded	1s
	2m 56s

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