Add support for Swift Package Manager to Succinct

[rgravina]

This adds support for Swift Package Manager!

• Adds a Package.swift which adds the Succinct and SuccinctTests targets. The SuccinctContainer and its tests were not added as they don't need to be exposed to users of the library.

• Added instructions in the Readme. Couldn't find a badge for the Swift Package Manager unfortunately!

• Inititally, there were build errors via SPM because the import <UIKit/UIKit.h> in Succinct.h wasn't having any effect. From what I could tell, there isn't an easy way to fix this (found some mention of it here). To get around this, imports from UIKit or Foundation were added to any file that used UIKit or Foundation (most of them). I also changed the import in Succinct.h to not import the FOUNDATION_EXPORT macro not from UIKit.h but another more specific header to make sure it's not possible to forget to import UIKit or Foundation. See this stack overflow answer for more details.

[theextremeprogrammer]

Thanks for this @rgravina ! I made two minor edits in a separate PR:

• Since you mentioned it, I added an SPM badge 👍🏻 Great idea!!
• I bumped the SPM install instructions to the top as this should be the best / most recommended approach for integration

I also discovered https://swiftpackageregistry.com/ and registered Succinct on there as well.

I'll cut a new release with SPM support and test this in the ToDoList app. Thanks again for your help with this!! 👏🏻🎉