Add Swift Package Manager support

[paulb777]

At WWDC 2019, Apple announced Xcode integration with Swift Package Manager. It would be great to be able to use it with OCMock.

[mRs-]

It would be awesome to use this. How is the current state of this? Can we get any information about this? #379 seems to got it working already. Is there any plan to provide this in the near future?

[erikdoe]

There's still the need for some clarification before #379 can be merged. It also has merge conflicts at the moment, but these are related to the clarifications.

This and #347 are the priorities of what I'm looking at. As always, I do this in my spare time, and I am not working on mobile apps in Swift so I need to learn about the concepts.

[JuhaO81]

Any active plans for SPM?