Do not implement `ZipEq::fold` (and change test) #862

Job	Run time
msrv	33s
check format	12s
doc	12s
check	12s
semver-checks	18s
check (--no-default-features)	17s
test	52s
check (--no-default-features --features use_alloc)	17s
check (--all-targets --all-features)	25s
All checks succeeded	1s
	3m 19s

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