psCharStrings: use floatToFixedToStr and strToFixedToFloat in T2CharString.toXML and fromXML
psCharStrings: use floatToFixedToStr and strToFixedToFloat in T2CharString.toXML and fromXML

Also, toXML signature must contain ttFont=None keyword argument, like
the rest of toXML methods elsewhere.

googlefonts/fonttools,fonttools/fonttools