add a benchmark for the _trusted setup_

as per title

changelog

• add a setup.rs benchmark which measures
  • the creation of a random setup
  • the serialization of a setup
  • the deserialization of a setup
• refactor plot.py a bit to
  • use argparse
  • take --bench to plot either linalg or setup results
  • write a complete plot_setup function
  • add a bit of documentation here and there

example results