Decimal and Foundation

[mredig]

I'm using this package in another package I'm working on and found myself needing to support Decimal.

While I was poking around, I also happened to notice that Foundation is very sparsely used, and therefore was easy to conditionalize for environments where Foundation might not be available (in theory, I think this might build for microcontrollers now without the Foundation features, but I have admittedly not tested it).

So this is obviously more than a small, itty bitty, incremental update, but has a few, only tangentially related features:

• Support for initializing from Decimal
  • I used a pretty naive solution, but it works
  • (requires Foundation, but...)
• Conditionally builds with Foundation features, when #if canImport(Foundation) is true. Otherwise, everything else is all pure Swift (citation needed)
• DRYed up the Data serialization and initializers, since they were right where I was working to conditionalize Foundation.
• DRYed up some of the FloatingPoint initializers, as a lot of the code was the same between the new Decimal inits and the existing ones.
• Added CustomDebugStringConvertible conformance, keeping it DRY with the existing Playground descriptions.

I completely understand if you don't want some, or even all of these changes and I'm happy to tweak the PR to accommodate, but I'll just have to use my own fork for my own needs then :)

[mredig]

Oh - also noteworthy - I added tests for the new stuff and confirmed all tests pass on my computer.

[tgymnich]

This is great. Thank you! Just one small thing.