Support USINGZ/user data in points

[msvbg]

This is related to #5, but I'm opening the PR here since I think it makes the most sense for the bindings to live here.

Clipper has a flag called USINGZ which adds a z field to the point structs, containing user data. This PR adds support for this USINGZ using a cargo feature flag.

Some notes:

• I decided to pre-generate bindings both with and without USINGZ
• I added a user_data argument to the callback, which enables a nicer API in the Rust bindings by allowing users to pass in a closure instead of a function pointer. I have another PR for this.