Add support for ip_in_core

[newAM]

However, I'm not sure how meaningful this PR will be until the changes land in stable, since library authors will implement the e-nal, and end users won't have the ability to pick and choose.

Crate authors can provide a pass-through to select this feature. Not a great solution, but at the same time if nobody uses ip_in_core then it likely wont get stabilized.

[ryan-summers]

Welp, let's try it out and see how it works! :)