Add a swift example

[liamappelbe]

Swift support (almost) already works, building on ObjC. Add an example.

Swift command to generate the dylib and objc wrapper header:

swiftc -c foo.swift -emit-objc-header-path foo.h -emit-library

Then we can run ffigen on foo.h to get the Dart bindings.

[dcharkes]

Should we have an language: swift to avoid confusion?

[liamappelbe]

no, because it's not actually going to parse swift files. The user has to generate the objc file for the swift library

[liamappelbe]

Maybe in future we can add language: swift, and just have ffigen invoke that swiftc command automatically. For now tho, I want to keep things simple to land it in time for 2.18, and just have ffigen treat it as a normal objc header.