Implement CValuesRef in interop

* Represent pointer parameters of C functions as `CValuesRef`.
* Represent struct value parameters as `CValue`.
* Implement some helper methods to make the new approach more useful.
* Migrate some code to new interop approach for pointers and values.

Also do not map to `String?`:
- pointers to 8-bit integers;
- pointers to non-const chars.

samples/tetris/tetris.kt

@@ -491,7 +491,7 @@ class Game(width: Int, height: Int, val visualizer: GameFieldVisualizer, val use
 
 }
 
-fun get_SDL_Error() = SDL_GetError()!!.asCString().toString()
+fun get_SDL_Error() = SDL_GetError()!!.toKString()
 
 class SDL_Visualizer(val width: Int, val height: Int): GameFieldVisualizer, UserInput {
     private val CELL_SIZE = 20