Consolidate targets across platforms

[abbeycode]

I recently discovered the "Supported Platforms" build setting. I should use that to make a single cross-platform framework and unit test bundle. This will make the project easier to maintain going forward.

• Update project's supported platforms
• Make UnzipKit target support all platforms
• Make UnzipKit Tests target support all platforms
• Update the build scripts to work with the same targets in each platform
• Once everything's working right for macOS and iOS with the unified targets, drop the iOS-specific targets

[abbeycode]

Done in PR #64