2D/3D code duplication

[zvxryb]

Some code was duplicated in the process of implementing 2D indices (issue #6).

Much of the code in index.rs (structs and SpatialIndex trait implementations) is already generated by a macro to avoid duplication. However, 2D/3D algorithms in geom.rs have a great deal of similarity and opportunities for improvement still exist.

[zvxryb]

Const generics and a number of other future Rust features might be helpful here, so I may postpone this until Rust generics are a bit more complete.