Sometimes use more genereric return type:
* _PointsShape.points() -> Iterable[Point]
* _Shape.pointsshapes() -> Iterable[PointsShapeT]
* _Shape.bounds() -> RectangularT
* MultiShape.shapes() -> Iterable[ShapeT]
* Generator[*, None, None] -> Iterable[*]

Privatise classes:
* SubLayouts -> _SubLayouts
* MaskShapesSubLayout -> _MaskShapesSubLayout

Privatise attributes:
* _Cell.circuits -> _Cell._circuits
* _Cell.layouts -> Cell._layouts
* _Layout._sublayouts -> _Layout._sublayouts

Signed-off-by: Staf Verhaegen <staf@fibraservi.eu>