add #simplify for Polygon and MultiPoligon when using CAPI

Hello, these add support for for using simplify without passing from FFI.

I added this because Heroku is bugged and doesn't support FFI on the Cedar stack, and they have no clue how to fix it (see: https://github.com/ffi/ffi/issues/344).

I'm not sure why in the MultiPolygon test I needed to use `@lenient_factory` (normal factory would return `nil`), maybe you can help me there so that I can possibly change it.

Please let me know if I have to change something!