- mostly wraps std::chrono so can inline much of it, which is potentially
  helpful when used for inner timings.

- add elapsedTime() method for direct cast to double and for
  naming similarity with wall-clock method.

Potential breaking change (minor):

- clockValue construct with a bool parameter is now simply tagged
  dispatch (value is ignored) and always queries the current clock
  value. This avoids needless branching.
  Since this constructor form has primarily been used internally (eg,
  clockTime), breakages in user code are not expected.