Use nanosecond timing from monotonic clock mtime for profiling

[nikswamy]

This PR uses the mtime package for measuring time, providing more accurate profiling. Previously, we were accumulating only millisecond timings in the profiling counters, which led to incorrect accounting.

Thanks to @LukeXuan for reporting the issue.

Note: This PR adds mtime to the list of opam dependences

[nikswamy]

check world succeeds too