[Speed] Latest enhancements of perf 0.8.1 and performance 0.3.1

[Victor Stinner]

Hum, so for an usability point of view, I think that the best to do is
to ignore the option if Python has a JIT.

On CPython, --duplicate makes sense (no?). So for example, the
following command should use duplicate on CPython but not on PyPy:

   python2 -m perf timeit '[1,2]*1000' --duplicate=1000 --compare-to=pypy

Victor