folly: print nice time suffixes in benchmark output

folly: print nice time suffixes in benchmark output

Summary: Print the times using "ms" for milliseconds, "us" for microseconds, etc.

Test Plan:
$ ./_bin/folly/test/benchmark_test
======================================================================
folly/test/BenchmarkTest.cpp              relative  time/iter  iters/s
======================================================================
bmFun                                                  3.36ns  298.01M
bmRepeatedFun                                          1.76ns  569.38M
----------------------------------------------------------------------
gun                                                    1.70ns  589.82M
----------------------------------------------------------------------
baselinevector                                         3.08us  324.87K
bmVector                                    13.64%    22.56us   44.32K
----------------------------------------------------------------------
superslow                                               1.00s  999.82m
======================================================================

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D546209