Renewing `Foundation.Decimal`

Hi all, Admittedly, the Foundation.Decimal type is the product of another time. It doesn't represent infinity or negative zero, for example, characteristics that are decidedly unlike those of standard IEEE types and that preclude conformance to the standard library's FloatingPoint protocol. We probably wouldn't design a decimal floating-point type from scratch today in a similar way. Still, this is the decimal floating-point type that Swift has today (now cross-platform!), and it continues to s...