Distribute a fast Batsim version

It would be interesting to propose a fast version of Batsim to users,
to improve the performance of simulations highly impacted by Batsim's overhead (very fast schedulers).

TODOs

• SimGrid
  • Evaluate the performance of the various context switch implementations.
  • Select other compilation flags (would be easy).
• Batsim
  • Separate internal and external checks (good'old assert vs enforce).
    This way we could disable all internal checks but keep the ones about the external files and processes.
  • Manage optimization in build system
• Distribution
  • Update the simgrid/batsim and batsim nix expressions