Possible optimization for isspace, isalpha, isdigit

[hordi]

Currently all these 3 functions take about 30% of execution time during parse a file.
If build local storage we can improve a file parsing ~15-20%. I have received reducing runtime from 35secs to 28secs for multiple big files processing.

https://github.com/hordi/tinyxml2