[SNIP]

Optimizer tag
-------------

Changes:

* Add ``sys.implementation.optim_tag`` (``str``): optimization tag.
  The default optimization tag is ``'opt'``.
* Add a new ``-o OPTIM_TAG`` command line option to set
  ``sys.implementation.optim_tag``.

Changes on ``importlib``:

* ``importlib`` uses ``sys.implementation.optim_tag`` to build the
  ``.pyc`` filename to importing modules, instead of always using
  ``opt``. Remove also the special case for the optimizer level ``0``
  with the default optimizer tag ``'opt'`` to simplify the code.
* When loading a module, if the ``.pyc`` file is missing but the ``.py``
  is available, the ``.py`` is only used if code optimizers have the
  same optimizer tag than the current tag, otherwise an ``ImportError``
  exception is raised.

Pseudo-code of a ``use_py()`` function to decide if a ``.py`` file can
be compiled to import a module::

    def transformers_tag():
        transformers = sys.get_code_transformers()
        if not transformers:
            return 'noopt'
        return '-'.join(transformer.name
                        for transformer in transformers)

    def use_py():
        return (transformers_tag() == sys.implementation.optim_tag)

The order of ``sys.get_code_transformers()`` matter. For example, the
``fat`` transformer followed by the ``pythran`` transformer gives the
optimizer tag ``fat-pythran``.

The behaviour of the ``importlib`` module is unchanged with the default
optimizer tag (``'opt'``).