Specialize `PeekNth::fold` #641

Job	Run time
check format	7s
msrv	27s
test	52s
check	7s
check (--no-default-features)	10s
check (--no-default-features --features use_alloc)	6s
check (--all-targets --all-features)	16s
All checks succeeded	1s
	2m 6s

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