Swift package manager supprort, and use Swift.Result

[klundberg]

I had to split the c and swift code into separate targets to make this work, since mixed-lang targets arent allowed in swiftpm, but it otherwise works well, and should be still cocoapods-compatible (based on a pod lib lint I ran).

Swift now has a built in result enum as well, which I've converted the project over to use instead of the previously custom result.