ENH: C extension: use const GEOSGeometry pointers instead of plain GEOSGeometry pointers where appropriate

Internal functions like `GeometryObject_FromGEOS` should accept `const` pointers instead of plain pointers, since those functions should not modify the GEOS Geometry.

Originally detected in #225 since we are getting `const` pointers back from GEOS functions but these were getting discarded on call to the PyGEOS C API.