Rust -> Swift interop

[lucasmerlin]

Because I was curious myself what the best way to call swift would be I've implemented three different methods:

use @_cdecl to export swift functions and call them from rust via extern "C"

Pros:

• Easy to implement
• Works great for simple function calls without arguments

Cons:

• Sharing anything more than ints and floats gets really complicated and will need lots of unsafe

via objc using objc2 crate

We can declare a objc "class" in some objc glue code and use the objc2::extern_class macro to declare it in rust.
Then we can call the swift code from our objective c code.

Pros:

• winit uses objc2 so there is a lot of example code
• Sharing complex data structures is possible and relatively easy
• icrate has rust bindings for a lot of ios apis

Cons:

• Needs a intermediary objc layer in the xcode project, can't call swift directly.

via swift-rs

We can create a swift package containing our ios swift code and link it to our rust code via swift-rs

Pros:

• No objc layer necessary
• Allows using strings and structs as args

Cons:

• iOS code needs to live in a separate Swift Package