Proper calculation of percent time

[bencopl]

The "% Time" column of the DrHook output format is calculated using the highest total walltime out of all the profiled regions. This is only valid if there are a pair of callipers encompassing the entire program.

DrHook tries to get around this by taking a note of the time at the first and last calls. We could implement something similar wherein we record the time between the profiler constructor and the final call to Profiler::stop but then we are making assumptions about where the constructor is called and it also becomes hard to take overhead into account.