Sped up jsonsl__str_fastparse
Sped up jsonsl__str_fastparse

This function is a significant hot spot, in my benchmarks. These changes
make it significantly faster.

* Made the loop use a sentinel instead of a count
* Made the flow of control more direct

mnunberg/jsonsl,snej/jsonsl,mnunberg/jsonsl,snej/jsonsl,snej/jsonsl,mnunberg/jsonsl,mnunberg/jsonsl,snej/jsonsl