A ppx extension to record module startup times.
Modules using ppx_module_timer (included in ppx_jane_kernel) now have
instrumentation to record their startup time. If the environment variable
PPX_MODULE_TIMER is set (to anything), each module records its startup time,
and before exiting the process prints out all of the module times in the order
they occurred.
If a file contains [@@@ppx_module_timer.pay_overhead_to_time_individual_definitions],
the individual definitions will be timed (at the cost of some minor extra overhead.)