Implement methods for conversions between Long and CPointer

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

@@ -161,6 +161,10 @@ val CPointer<*>?.rawValue: NativePtr
 
 fun <T : CPointed> CPointer<*>.reinterpret(): CPointer<T> = interpretCPointer(this.rawValue)!!
 
+fun <T : CPointed> CPointer<T>?.toLong() = this.rawValue.toLong()
+
+fun <T : CPointed> Long.toCPointer(): CPointer<T>? = interpretCPointer(nativeNullPtr + this)
+
 /**
  * The [CPointed] without any specified interpretation.
  */

Interop/Runtime/src/native/kotlin/kotlinx/cinterop/Varargs.kt

@@ -78,7 +78,7 @@ fun callWithVarargs(codePtr: NativePtr, returnValuePtr: NativePtr, returnTypeKin
     // All supported arguments take at most 8 bytes each:
     val argumentsStorage = argumentsPlacement.allocArray<LongVar>(totalArgumentsNumber)
     val arguments = argumentsPlacement.allocArray<CPointerVar<*>>(totalArgumentsNumber)
-    val types = argumentsPlacement.allocArray<CPointerVar<*>>(totalArgumentsNumber)
+    val types = argumentsPlacement.allocArray<COpaquePointerVar>(totalArgumentsNumber)
 
     var index = 0
 
@@ -87,7 +87,7 @@ fun callWithVarargs(codePtr: NativePtr, returnValuePtr: NativePtr, returnTypeKin
         val typeKind = convertArgument(argument, isVariadic = isVariadic,
                 location = storage, additionalPlacement = argumentsPlacement)
 
-        types[index] = interpretCPointer<COpaque>(nativeNullPtr + typeKind.toLong())
+        types[index] = typeKind.toLong().toCPointer()
         arguments[index] = storage
 
         ++index