csharp: Allow improved cbs in eina comparators
Instead of receiving a callback that compares two intptrs (which is
still used internally), expose a more C#-friendly delegate which is
wrapped into the native one.
Test Plan: run test suite
Reviewers: felipealmeida, segfaultxavi, brunobelo
Reviewed By: felipealmeida
Differential Revision: https://phab.enlightenment.org/D10253