Cleanup Objective-C interop code

[frenzibyte]

• Moves NSString/NSImage conversion methods away from Cocoa class
• Defines NSString and uses stringWithCharacters:length: rather than alloc & initWithCharacters:length:
  • General rule of thumb in Objective-C to never use alloc and init if there's a "convenient constructor" available.
• Uses LibraryImport instead of DllImport (optional but why not)
• Moves code that can be shared with iOS to a different namespace and renames Cocoa to Interop.

I've also explored the path of using native SDKs offered by .NET but it was not a simple change and a lot needs to be reconsidered, so I'm not looking into that any further at the moment.

[smoogipoo]

To confirm, have you tested this on a release mode build of o!f on iOS (i.e. LLVM full opts)?

[frenzibyte]

Hmm, iOS did not come to my mind while working on this, although I did touch it in the next PR...I'll test on release mode and confirm.

[frenzibyte]

I suppose you've tested this already, but I just got around doing that and tested this alongside the follow-up PR as well (#6475). All working correctly on a release build of iOS.