* Refine stubs for Python 2's decimal module
The decimal module for Python 2 was relatively incomplete, unlike the
decimal module for Python 3. This commit copies the relevant type
signatures from Python 3's decimal module to Python 2's.
There was a lot of code in both stubs and it wasn't clear to me if it
was safe to merge the two modules together, so I refrained from doing
so.
* Allow comparisions with Decimals and Floats
This commit loosens the types for Decimals to allow comparisons like
`Decimal('3.14') < 4.2`. Previously, you could compare decimals with
only other decimals or ints.
