A ppx extension to record module startup times.
Modules using ppx_module_timer (included in ppx_jane) 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.)