Notes on GSoC Decimal arithmetic

[xwu (Xiaodi Wu)]

Probably useful to link to your earlier reply as well with a bunch of useful information:

[GSoC 2021] Swift Numerics: Decimal64

Decimal128 isn't really harder than 64, but it requires more machinery be built to support it. We already have 64 bit integer arithmetic (including double-width multiply and divide) in Swift, which makes building Decimal64 relatively straightforward, once you implement some basic primitives for packing and unpacking the representation and doing rounding. Decimal128 requires building a bunch of primitives for Int128 first. I will probably have already done this by the time a GSoC project gets s…