Use builder pattern for BezPath

Currently creating bezpaths (rect in this example) and applying transform looks like this: ```rust let mut path = BezPath::new(); path.move_to((0.,0.)); path.line_to((10.,0.)); path.line_to((10.,10.)); path.line_to((0.,10.)); path.close(); path.apply_affine(Affine::IDENTITY); ``` but using builder pattern one could do: ```rust let path = BezPath::new() .move_to((0.,0.)) .line_to((10.,0.)) .line_to((10.,10.)) .line_to((0.,10.)) .close() .apply_affine(Affine::IDENTITY); ``` signature of methods would be `move_to(&mut self, pt: impl Point) -> &mut Self`. My main motivation is to make applying affine easier.