`OrderedFloat` and `NotNan` should implement `geo::HasKernel` (behind feature)

[MultisampledNight]

The geo crate uses this to determine what kernel method for use for this type. Since OrderedFloat and NotNan are basically only wrappers for f64 and f32 (and other floats), we can safely assume that's always RobustKernel. Or even make the implementation of it generic depending on whether or not the internal type implements HasKernel.

I need this like right now (not to be merged, just publicly accessible), so I'll go ahead and setup a PR.