Drop support for 32-bit platforms

[masonmark]

Dropping support for 32-bit will allow us to use Int for all API data structures (which include 64-bit integers). Why is that desirable? Because it will let us use JSONSerialization to decode arbitrary JSON data structures (we will move Codable for serialization of objects/structs, and known JSON structures).

The JSONSerialization implementation on Linux does not support Int64 regardless of platform, but it does support 64-bit Int on 64-bit platforms.

At this point, I think we still want to use JSONSerialization when receiving an unknown/arbitrary structure as JSON.

[masonmark]

The JSONDecoder.UnevaluatedJSON mentioned in this post by Itai Ferber (main guy behind Codable) sounds intriguing... but also far in the future (post-Swift 5).

Also interesting: I read on that same forum that Codable uses JSONSerialization under the hood, so it's probably a safe bet that JSONSerialization will be fairly well-maintained going forward (it's had quite a few issues on Linux over the life of this project, but has steadily advanced...).

[masonmark]

Hmm, we have a unit test for our assumption that "JSONSerialization cannot decode an Int64" that I just noticed fails on Swift 4.1 beta and Ubuntu 16.04... which means it can decode that now. I should go back and trace when that assumption became invalid.

I have no idea what my own comment above means: "The JSONSerialization implementation on Linux does not support Int64 regardless of platform, but it does support 64-bit Int on 64-bit platforms." (Did I mean 64-bit Apple platforms? Is Linux JSONSerialization's ability to decode Int64 actually new?)