monitor proofs for performance regressions

[lsf37]

It'd be nice to know (automatically) if and which proof sessions get slower or faster over time. Also the total time for test runs.

The weekly clean tests should produce reasonably reliable timing information modulo some noise. We could record that timing info over time, and run a script similar to the seL4 performance regression script that plots performance over time as well as raises an alarm on significant jumps.