Performance Optimizations using std::move and constexpr

[evanwporter]

Small little performance optimizations

• std::move within toString method. Move instead of copies result
• Define DecimalFactor at compile time rather than run time

[evanwporter]

Note that this may conflict with #54 (without a small adjustment to #54), but seeing as that has yet to be merged...

[evanwporter]

FYI: Copying becomes more computationally costly as the string gets longer.

[vpiotr]

Looks OK, thanks!