Make `Product` lazy #599

Job	Run time
test	56s
check	6s
check format	7s
check (--no-default-features)	7s
msrv	1m 11s
check (--no-default-features --features use_alloc)	9s
check (--all-targets --all-features)	21s
All checks succeeded	1s
	2m 58s

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