Better performance measurement infrastructure

It would be great to have a more fine-grained breakdown of the performance of verification.

Ideas:

Add Time and Set Ltac Profiling around the different parts of function verification and have them automatically be inserted by a flag to the frontend
Have a script to automatically compare the output between different commits
Integrate more information with http://coq-speed.mpi-sws.org/
- Not sure how since the only reliable number is instruction count, not time, see e.g. this diff
- Can one somehow send signals to perf?