extract the code from fixedToStr into its own function (defined in roundTools), to be used for serializing fractional F2Dot14 angles for COLRv1 PaintRotate etc.
Moving out of CFF code, to be used in VariationModel(). Part of https://github.com/fonttools/fonttools/issues/2213
Code relying on old place (fixedTools.otRound) still works.