Specialize `RepeatN::[r]fold` #635

Job	Run time
check format	8s
test	51s
check	8s
msrv	34s
check (--no-default-features)	13s
check (--no-default-features --features use_alloc)	20s
check (--all-targets --all-features)	17s
All checks succeeded	1s
	2m 32s

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