Rewrite `MultiProduct` #719

Job	Run time
check	7s
doc	12s
test	53s
check (--no-default-features)	8s
msrv	38s
check (--no-default-features --features use_alloc)	7s
check (--all-targets --all-features)	17s
check format	10s
All checks succeeded	1s
	2m 33s

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