Specialize `Interleave[Shortest]::fold` #699

Job	Run time
check format	6s
msrv	37s
doc	7s
check	15s
test	51s
check (--no-default-features)	6s
check (--no-default-features --features use_alloc)	6s
check (--all-targets --all-features)	18s
All checks succeeded	1s
	2m 27s

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