SPM-compatible project structure. Linux support. Better podfile. CocoaPods deploy action

[ypopovych]

Hello.

I did a couple of updates to the repo and fixes:

1. Updated project structure to be more SPM-like
2. Moved MG code to MGTools target and added it to SPM so it can be run built on Linux
3. Moved MG Tests to own target
4. Added tests to the Podfile
5. Added CocoaPods tests run phase to CI.
6. Added Linux tests to CI.
7. Fixed and added tests for: BigInt initializer crashes on large integers #76
8. Fixed == and != for negative integers and added tests for it.
9. Added deploy workflow to the CI for CocoaPods repo push.

[mkrd]

@ypopovych Hey, thank you so much for your contribution! Looks good on first sight, I will review it in detail later. @twodayslate, I would also appreciate your review :)

[ypopovych]

CocoaPods GitHub action workflow will need COCOAPODS_TRUNK_TOKEN secret variable in the repository. How to obtain it: https://fuller.li/posts/automated-cocoapods-releases-with-ci/