Replaced IPNetwork's raw pointer usage with smart pointers.

[Dimi1010]

Main points of this PR are the replacement of raw pointers, new and delete calls in IPNetwork with usages of std::unique_ptr<T> and a backported implementation of std::make_unique.

[Dimi1010]

Also, just asking, is it desirable for internal detail namespaces to be documented in the public API documentation?

[tigercosmos]

Also, just asking, is it desirable for internal detail namespaces to be documented in the public API documentation?

yes, we should document everything.