Added Xcode project with shared schemes to enable Carthage compatibility

[gwikiera]

Carthage needs shared xcschemes to be able to fetch dependencies.
This PR adds xcproj with shared xcschemes for all supported platforms.

[klundberg]

Thanks for this PR, it should allow this to be used with carthage easily!

I have a question and some requests though:
For my question; I'm curious what the use case for using carthage here is, when carthage frameworks and swift packages can be both mixed into the same project? Will this be a transitive dependency with another carthage library, or will the frameworks be cached somewhere or something?

As for my concerns, we don't use carthage internally, and I'm worried about the added maintenance of an xcodeproj file with many targets. If we make or accept changes to the swift package, we'd need to make sure to reflect any changes to file structure or file additions in the project too, which is less than ideal. Could you change this PR to instead say, include a workspace that has the swift package as a reference, and also an xcode project with a framework target that just statically links the swift package? I think that should work with carthage, though that's worth a test. If we do this, then the project will just be an empty shell that will need minimal maintenance.

Secondly, would it be possible to condense the frameworks down to one multi-platform framework target/scheme? I believe this should be possible with some changes to the build settings of a target, and as I recall having used carthage in the past it should work with carthage as well I believe? That'd make maintenance of the project file even easier since build/target settings won't need to change for four targets, only one if changes need to be made.

Thanks!

[klundberg]

I've decided I'll merge this as is, and explore those enhancements later so that folks can start using this with carthage sooner :)