I like this.  The speedup is from reduction of allocations+GC

I think you need to fall back to CLOCK_REALTIME on systems w/o
CLOCK_MONOTONIC, though.  Based on my reading of process.c,
CLOCK_REALTIME is always available.  So something like this:

	if defined?(Process::CLOCK_MONOTONIC)
	  BENCHMARK_CLOCK = Process::CLOCK_MONOTONIC
	else
	  # Ruby may use gettimeofday to emulate:
	  BENCHMARK_CLOCK = Process::CLOCK_REALTIME
	end

	def realtime # :yield:
	  r0 = Process.clock_gettime(BENCHMARK_CLOCK)
	  yield
	  Process.clock_gettime(BENCHMARK_CLOCK) - r0
	end