[psCharStrings] split width-extracting part of T2OutlineExtractor into T2WidthExtractor
[psCharStrings] split width-extracting part of T2OutlineExtractor into T2WidthExtractor

We shall reuse it as base class for the _DehintingT2Decompiler in subset module,
as we need to save the width in order to re-insert it after dropping the hints.

googlefonts/fonttools,fonttools/fonttools