... to give it a richer set of operations, such as len() to get the number of components and an implementation of .inverse_family() to allow looking up the key of a given component (LinearFunction instance).

This requires making AbstractFamily and FiniteFamily Cython classes.

Depends on #34839

CC: @yuan-zhou @tscrim @mantepse

Component: linear programming

Author: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/31750