OK, here's a patch. Specifics:

  o Except for the number methods, decimal.py and _decimal should behave
    identically now.

  o _decimal actually requires an additional context arg in same_quantum(),
    compare_total(), compare_total_mag() and copy_sign(). This is for raising
    InvalidOperation if the second operand is a very large integer and cannot
    be converted exactly.

    I've added the context arguments to the Python version to keep compatibility.

  o For some reason decimal.py uses a context arg in canonical(), so I've
    added it to the C version.

  o localcontext(ctx=None) is pretty ugly. Perhaps we should make the
    backwards incompatible switch to localcontext(context=None). I don't
    care a lot though.

  o I'd like to update the docstrings and the documentation in a separate
    patch.

  o The important idiom c.power(modulo=None, b=9, a=3) is now supported. :)