Decimal IEEE 754-2019

[davdroman (David Roman)]

I might be out of my depth here, but I think conforming Decimal to ExpressibleByFloatLiteral perpetuates a poor design choice in the original type, which allows for things like these to happen:

let foo: Decimal = 3.133
print(foo) // 3.132999999999999488

Instead, we should either completely bar Decimal from being expressible by literal, or have Swift implement a native ExpressibleByDecimalLiteral.

This has been extensively discussed in other threads, like:

Decimal32, Decimal64, and Decimal128 Design Approach

Here's the updated Decimal32 interface and top-level implementation. Names have been changed as suggested. I'm currently not showing the data encoding structure used to store the raw data to avoid confusion (please see the first post for more details). More comments have been added. BTW, the lack of ExpressibleByFloatLiteral is sorely missed (even though it is not an exact representation). It definitely makes initializing Decimal numbers cumbersome. We really need something like Expressibl…