Mirror the read API to create a mesh file

[MJohnson459]

While debugging it was useful to generate a mesh file that could be viewed and shared easily.

This PR adds the mirror functions for writing a Mesh to a file. Given it's purpose is for debugging I didn't spend too long making the API nicer and just copied what was there.

        let file: PolyanyaFile = mesh.into();
        file.to_file(&"nav_mesh.poly");