Networked swift bindings

[za-creature]

This is a complete overhaul of the swift bindings with the following major changes:

• implemented the loopback authentication protocol developed on the service branch
• exposed all (known) RPCs exposed by the current library
• all get* methods have been replaced with actual async getters
• exposed subscriptions via AsyncStream, semantically equivalent to the dart api
• memory management is now done implicitly via reference counting, the close methods are not exposed (but are called internally on deinit)
• the bindings are now free from force unwraps (except one instance where Apple's API is weird)
• ported dart tests with a current coverage of ~75%
• imported documentation from dart wherever possible

Other QoL changes:

• moved the relevant post-checkout script from the 'app' repo here (init.sh)
• updated the build tools to support swift build and command line coverage generation (cov.sh)
• renamed from OuisyncLib to Ouisync (the name collision with the UI is resolved by renaming its module to Runner)