Avoid string allocation in tree lookup for every request

[gallir]

path := string() allocates a new string for every call, use
the same b2s() function as in fasthttp to avoid the allocation.

[coveralls]

Coverage increased (+0.008%) to 98.253% when pulling d098ad7 on gallir:b2s into 979d6e5 on buaazp:master.

[savsgio]

Hi @gallir,

The official fasthttp organization is maintaining this repo in https://github.com/fasthttp/router.
It has some improvements and optimizations in performance. I recommend you use it.

Thanks.

[savsgio]

path := string() allocates a new string for every call, use
the same b2s() function as in fasthttp to avoid the allocation.

This change is did in official router repository.