add a benchmark for the _trusted setup_ (!52)

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
![Figure_1](/uploads/ea4bddc5c0c426d0824bad55e2e2e5aa/Figure_1.png)