sequences.product: use some unsafe to make it faster.
sequences.product: use some unsafe to make it faster.

dch/factor,bpollack/factor,slavapestov/factor,dch/factor,bpollack/factor,bpollack/factor,dch/factor,slavapestov/factor,tgunr/factor,slavapestov/factor,slavapestov/factor,AlexIljin/factor,AlexIljin/factor,bjourne/factor,factor/factor,bjourne/factor,sarvex/factor-lang,tgunr/factor,bjourne/factor,factor/factor,sarvex/factor-lang,tgunr/factor,AlexIljin/factor,slavapestov/factor,nicolas-p/factor,slavapestov/factor,bpollack/factor,factor/factor,mrjbq7/factor,factor/factor,tgunr/factor,nicolas-p/factor,bjourne/factor,nicolas-p/factor,mrjbq7/factor,dch/factor,bjourne/factor,sarvex/factor-lang,sarvex/factor-lang,factor/factor,mrjbq7/factor,sarvex/factor-lang,dch/factor,sarvex/factor-lang,mrjbq7/factor,nicolas-p/factor,bpollack/factor,AlexIljin/factor,bjourne/factor,tgunr/factor,factor/factor,bjourne/factor,mrjbq7/factor,AlexIljin/factor,dch/factor,nicolas-p/factor,bpollack/factor,sarvex/factor-lang,AlexIljin/factor,tgunr/factor,bpollack/factor,nicolas-p/factor,mrjbq7/factor,slavapestov/factor,AlexIljin/factor,nicolas-p/factor