Byproducts of aborted attempt to refine 'parent' proxy semantics.

Byproducts of aborted attempt to refine 'parent' proxy semantics.
Mostly cleanup of mpy_importer.mpy_parse().

python/m5/__init__.py:
    Move panic() up to top in case we want to use it
    in mpy_importer (though I ended up not doing that
    after all).
python/m5/config.py:
    Add a couple of comments and a check for expressions
    like parent.any.foo (which is illegal).

--HG--
extra : convert_revision : dfc99ac9b1a2d91a736ca0b773b6d3c528a4f3cc