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/basics/0.kt

@@ -2,7 +2,7 @@ import sysstat.*
 import kotlinx.cinterop.*
 
 fun main(args: Array<String>) {
-    val statBuf = nativeHeap.alloc<statStruct>()
+    val statBuf = nativeHeap.alloc<stat>()
     val res = stat("/", statBuf.ptr)
     println(res)
     println(statBuf.st_uid)

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;

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

@@ -24,11 +24,6 @@
     return self;
 }
 
--(void)hello {
-    CPrinter* printer = [[CPrinter alloc] init];
-    [self helloWithPrinter:printer];
-}
-
 -(void)helloWithPrinter:(id <Printer>)printer {
     NSString* message = [NSString stringWithFormat:@"Hello, %@!", self.name];
     [printer print:message.UTF8String];
@@ -40,6 +35,14 @@
 
 @end;
 
+@implementation Foo (FooExtensions)
+
+-(void)hello {
+    CPrinter* printer = [[CPrinter alloc] init];
+    [self helloWithPrinter:printer];
+}
+@end;
+
 void replacePairElements(id <MutablePair> pair, int first, int second) {
     [pair update:0 add:(first - pair.first)];
     [pair update:1 sub:(pair.second - second)];