Specialize `ZipLongest::rfold` #698

Job	Run time
check format	10s
msrv	42s
check	13s
doc	7s
check (--no-default-features)	11s
test	48s
check (--no-default-features --features use_alloc)	8s
check (--all-targets --all-features)	24s
All checks succeeded	1s
	2m 44s

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