Support user data with USINGZ

[msvbg]

Sister PR to tirithen/clipper2c-sys#6. Adds a usingz cargo feature to support user data in points.

I think the main complication is the double Box dance to be able to pass Rust closures as a C callback. I think this is a nicer API than just a function pointer, which would also leak clipper2c-sys internal types.

[msvbg]

Note: fix this before merging

[msvbg]

One problem with this PR is that it breaks e.g. scale_around_point since it uses the Point::new API instead of Point::new_with_z. Maybe the cleanest way to solve this is to have two types, one Point type and a PointZ type or something instead of putting #[cfg(feature = "usingz")] everywhere around the code base.

[songhuaixu]

One problem with this PR is that it breaks e.g. scale_around_point since it uses the Point::new API instead of Point::new_with_z. Maybe the cleanest way to solve this is to have two types, one Point type and a PointZ type or something instead of putting #[cfg(feature = "usingz")] everywhere around the code base.

I think using PointZ might be better