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.

InteropExample/src/main/kotlin/main.kt

@@ -24,7 +24,7 @@ fun main(args: Array<String>) = memScoped {
     }
     val error = errorRef.value
     if (error != null) {
-        println(error.asCString().toString())
+        println(error.toKString())
         return
     }