Move timing functions to the Debug.Stats infrastructure.

As a consequence, the --print-timing option no longer exists and it is replaced with --debug=stats. This patch also performs the following modifications:

• It removes uses of Unix.times, as it breaks TryWhy3.
• It properly records timing in case of exception.
• It cleans the Debug.Stats code a bit.

@mmanighe Please check if the feature still works as intended. In particular, the get_timings function no longer exists, but you perhaps need it for some external code?