research: alternative to `string` for `decimal` conversion

[DaveSkender]

I had a quick test. And this way has max 3x performance gain!

    if cs_decimal is not None:
        return CsDecimal.ToDouble(cs_decimal)

But few tests fail due to precision problem

Originally posted by @LeeDongGeon1996 in #386 (comment)

[DaveSkender]

This is worth exploring IMO given the risk vs. reward tradeoff; though, we should check it out to see if precision loss from converting through double to decimal is significant compared to current string based conversion. See prior discussion on Precisions v. Performance of decimal vs double.