Do not implement `ZipEq::fold` (and change test) #860

Job	Run time
check format	14s
check	19s
msrv	33s
check (--no-default-features)	16s
doc	15s
semver-checks	2m 28s
check (--no-default-features --features use_alloc)	19s
test	56s
check (--all-targets --all-features)	26s
All checks succeeded	1s
	5m 47s

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