Do some work on wrenCall().

- Add an explicit va_list version. That lets variadic functions
  forward to it.
- Fix a GC bug in wrenCall() with return values.
- Make the call API test not re-enter the VM.

test/api/main.c

@@ -36,7 +36,6 @@ static WrenForeignMethodFn bindForeignMethod(
   strcat(fullName, ".");
   strcat(fullName, signature);
 
-  REGISTER_METHOD(call, call);
   REGISTER_METHOD(foreign_class, foreignClass);
   REGISTER_METHOD(returns, returns);
   REGISTER_METHOD(value, value);
@@ -58,6 +57,10 @@ static WrenForeignClassMethods bindForeignClass(
   return methods;
 }
 
+static void afterLoad(WrenVM* vm) {
+  if (strcmp(testName, "call") == 0) callRunTests(vm);
+}
+
 int main(int argc, const char* argv[])
 {
   if (argc != 2)
@@ -74,7 +77,7 @@ int main(int argc, const char* argv[])
   strcat(testPath, testName);
   strcat(testPath, ".wren");
 
-  setTestCallbacks(bindForeignMethod, bindForeignClass);
+  setTestCallbacks(bindForeignMethod, bindForeignClass, afterLoad);
   runFile(testPath);
   return 0;
 }