move surf::As to s4u::As

move surf::As to s4u::As

- The AS are now usable by the user (retrieve the root AS from s4u::Engine)
- This allows to kill a bunch of C->C++ wrapper functions: surf_AS_*
- SimDag now needs a s4u::Engine, which initializes Simix. But beside
  the initialization cost, this should not harm the simdag performance.

The change is a bit brutal, and some more work will be needed to finish it.

- RoutingPlatf should be properly integrated to the s4u::Engine
- xbt containers should be changed into std:: ones
- we should do one header per class and stop all-in-one headers
- various improvements that I don't clearly see yet

But things are improving and getting a shape, I'm happy.