Add a Package.swift manifest

I'd like to add a package dependency to my app in Xcode and enter this repository's URL.

Of course, when I try that now, I get the error:

https://github.com/signalapp/libsignal-protocol-c.git has no Package.swift manifest for version 2.3.3

If this repo had a Package.swift file, then integrating it into my iPhone app would be much easier.

Subtasks

Remove the CMakeModules/iOS.toolchain.cmake file
Update the README
- Change "beyond CMake" to "beyond CMake or Swift Package Manager"
- Remove the "iOS" bullet point

Fixing this issue would obsolete #16 & #34.

Related Issues

signalapp / libsignal-protocol-c