Improve function types support in interop

* Support `typedef` to function type without pointer;
* Support 64-bit integers in function types;
* Generate opaque function type on JVM if not supported.

Fix other minor bugs.

Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Types.kt

@@ -281,10 +281,12 @@ class CFunction<T : CFunctionType>(override val rawPtr: NativePtr) : CPointed
 
 /**
  * The pointer to [CFunction].
+ * TODO: remove.
  */
 typealias CFunctionPointer<T> = CPointer<CFunction<T>>
 
 /**
  * The variable containing a [CFunctionPointer].
+ * TODO: remove.
  */
 typealias CFunctionPointerVar<T> = CPointerVarWithValueMappedTo<CFunctionPointer<T>>