Improve Xcode support #459

There is a bunch of Rust tools out there that ease access to (parts of) Xcode:

• Cargo runner (mine): cargo-apple-runner
• Create a new Xcode project: cargo-xcode
• Create new Xcode project and launch it in the simulator / on device:
  • cargo-dinghy
  • cargo-mobile
  • cargo-mobile2
  • crossbundle / crossbow
  • dx serve --platform ios
• Create application bundles:
  • cargo-bundle / tauri-bundler
  • xbuild
  • tugger
• Launch Instruments: cargo-instruments
• Create fat binaries: cargo-lipo
• Make Rust packages available to Objective-C/Swift: cargo-cocoapods
• Create and launch a .app bundle automatically: fruitbasket
• Lower-level tools for Apple-specific stuff: apple-platform-rs, apple-bundle

But all of them currently feel quite clunky, or are missing a lot of functionality.

If we actually desire at least somewhat competitiveness with Swift (#429), it needs to be easy to:

• Use storyboards/nib files.
• Use assets.
• Configure application information (Info.plist).
• Use advanced debugging utilities (main thread checker, UI hierarchy viewer, metal shader debugger, ...).
• Bundle, sign and publish to the app store.

All of these are certainly possible to do outside of Xcode, and we should strive towards that eventually, but it's going to take a lot of time for the ecosystem to get there! So instead, we should start by making it easier to work with Rust projects inside Xcode!

Perhaps a path forward would be some combination of rust-xcode-plugin / rust-xcode-langspec, a Source Editor Extension and cargo-xcode? Ideally, we'd want full LSP-like support, and probably eventually some kind of IBAction/IBOutlet support, but having a solid way to run Rust applications from inside Xcode would be a huge step!

Let's also remember that Rust is not the only ecosystem that has this problem, perhaps we can learn from what others are doing? Mentioning XcodeGen here.