Add cache in charsArray()
Add cache in charsArray()

This is static but confined to this function block,
so this doesn't introduce any side effect.

The performance gain is really worth it.

takobell/Stringy,taohaoge/Stringy,ycms/Stringy,danielkesselberg/Stringy,ihsanudin/Stringy,TCB13/Stringy,wallacesilva/Stringy,danielstjules/Stringy,Big-Shark/Stringy,bocharsky-bw/Stringy,sofyansitorus/Stringy,marktopper/Stringy,voku/Stringy