Specialize `Positions::[r]fold` #622

Job	Run time
test	57s
check	11s
msrv	25s
check (--no-default-features)	12s
check format	17s
check (--no-default-features --features use_alloc)	12s
check (--all-targets --all-features)	24s
All checks succeeded	1s
	2m 39s

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