Decide on a unified convention for using vs typedef. (was ACTS-466)

Original author Hadrien Benjamin Grasland @hgraslan

Modern C++ is full of near-equivalent ways to do the same thing. One example is typedefs versus using declarations.

We probably want to settle on a single way of doing these. I think that of those, using is a little bit more powerful than typedef as it works better with templates and has a syntax which is more consistent with variable assignments.

Maybe we can ask one of our clang-tools to help us at the task of migrating towards one form and sticking with it in the future?