sequences: change trim-head to not call length unless it needs to.
sequences: change trim-head to not call length unless it needs to.

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