Extend traits class to support custom logic after swaps and rotations

Hi,

The Ygg library of intrusive data structures (https://github.com/tinloaf/ygg) allows custom logic to be called after node swaps and rotations by implementing the node traits class interface. For example:

- Node traits: https://github.com/tinloaf/ygg/blob/ea8f04fdf8e5d19e048ba9f8d130bfda75863756/src/rbtree.hpp#L120
- Implementation of the left rotation: https://github.com/tinloaf/ygg/blob/ea8f04fdf8e5d19e048ba9f8d130bfda75863756/src/rbtree.cpp#L318

Do you see it possible to allow similar functionality in Boost.Instrusive by extending the traits class interface similarly to as in Ygg?