Character serialization diff-friendly

If you keep your anathema repository under version control dealing with random serialization order it's a pain (CharmGroups, Charms and AdditionalModels).
In this way it's easier to understand what is changed in your character with a simple diff.


I first developed this on version 5.0.1 ( https://github.com/nolith/anathema/commit/79532298c017677a268f1a6bc887b363bfa540a5 ) but I imagined that you would have liked a master based pull-request, so I rebased all my work on master.