Allows inheritance, reduces duplication, removes the awkward "replaces" of the phases, allows verification.
A con is that we have more files now (a tech file per civ).
Stuff to do first:
- unit_elephant* to mixin.
- Civ-bonuses as player-auras?
- Engine function to load all templates in a folder.
Modders can migrate part of their techs (it seems not all (yet)) with P267.