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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. :)