* src/misc/timer.hh (stopwatch): New class, implemented on top
of C++11's std::chrono::high_resolution_clock.
* src/bin/dstar2tgba.cc, src/bin/ltl2tgba.cc, src/bin/ltlcross.cc:
Use it in lieu of gethrxtime(), so we do not need to distribute
gethrxtime anymore.