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Finishes addressing issue #424.

In the end this was a very neat refactor that now makes things a lot
more consistent across the py code base.  It allowed some
simplifications in certain places, now that everything is a dict object.

Also converted builtins tables to dictionaries.  This will be useful
when we need to turn builtins into a proper module.

That's how CPython has it, in particular, "import __main__" should work.

So far just includes "open" function, which should be supplied by a port.

TODO: Make the module #ifdef'ed.

Addresses issue #388.

Only exceptions that are actually used are left prefedined.  Hierarchy
is still there, and removed exceptions are just commented out.

They still exist in commented-out form in objexcept.c if they are ever
needed.

Comes with some refactoring of code and renaming of files.  All modules
are now named mod*.[ch].