Support interop modularity

* Add list of included headers into the manifest
* Implement importing "foreign" type declarations
* Implement forward declarations more natively in the compiler
* Represent Objective-C categories as Kotlin extension methods and
  properties

Also:
* Do some refactoring in stub generation
* Call bridges directly in stubs for Objective-C properties

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

@@ -7,10 +7,13 @@
 
 @interface Foo : NSObject
 @property NSString* name;
--(void)hello;
 -(void)helloWithPrinter:(id <Printer>)printer;
 @end;
 
+@interface Foo (FooExtensions)
+-(void)hello;
+@end;
+
 @protocol MutablePair
 @required
 @property (readonly) int first;