Implement very basic Objective-C interop

Also refactor StubGenerator

backend.native/tests/interop/objc/smoke.kt

@@ -0,0 +1,55 @@
+import kotlinx.cinterop.*
+import objcSmoke.*
+
+fun main(args: Array<String>) {
+    autoreleasepool {
+        run()
+    }
+}
+
+fun run() {
+    val foo = Foo()
+    foo.hello()
+    foo.name = "everybody"
+    foo.helloWithPrinter(object : NSObject(), PrinterProtocol {
+        override fun print(string: CPointer<ByteVar>?) {
+            println("Kotlin says: " + string?.toKString())
+        }
+    })
+
+    Bar().hello()
+
+    val pair = MutablePairImpl(42, 17)
+    replacePairElements(pair, 1, 2)
+    pair.swap()
+    println("${pair.first}, ${pair.second}")
+}
+
+fun MutablePairProtocol.swap() {
+    update(0, add = second)
+    update(1, sub = first)
+    update(0, add = second)
+    update(1, sub = second*2)
+}
+
+class Bar : Foo() {
+    override fun helloWithPrinter(printer: PrinterProtocol) = memScoped {
+        printer.print("Hello from Kotlin".cstr.getPointer(memScope))
+    }
+}
+
+@Suppress("CONFLICTING_OVERLOADS")
+class MutablePairImpl(first: Int, second: Int) : NSObject(), MutablePairProtocol {
+    private var elements = intArrayOf(first, second)
+
+    override fun first() = elements.first()
+    override fun second() = elements.last()
+
+    override fun update(index: Int, add: Int) {
+        elements[index] += add
+    }
+
+    override fun update(index: Int, sub: Int) {
+        elements[index] -= sub
+    }
+}