Add test for user data.

Also moved the VM parameter in the error callback to be first like it
is in other callbacks.

test/api/user_data.c

@@ -0,0 +1,51 @@
+#include <string.h>
+
+#include "user_data.h"
+
+static const char* data = "my user data";
+static const char* otherData = "other user data";
+
+static void test(WrenVM* vm)
+{
+  WrenConfiguration configuration;
+  wrenInitConfiguration(&configuration);
+  
+  // Should default to NULL.
+  if (configuration.userData != NULL)
+  {
+    wrenSetSlotBool(vm, 0, false);
+    return;
+  }
+  
+  configuration.userData = (void*)data;
+  
+  WrenVM* otherVM = wrenNewVM(&configuration);
+  
+  // Should be able to get it.
+  if (wrenGetUserData(otherVM) != data)
+  {
+    wrenSetSlotBool(vm, 0, false);
+    wrenFreeVM(otherVM);
+    return;
+  }
+  
+  // Should be able to set it.
+  wrenSetUserData(otherVM, (void*)otherData);
+  
+  if (wrenGetUserData(otherVM) != otherData)
+  {
+    wrenSetSlotBool(vm, 0, false);
+    wrenFreeVM(otherVM);
+    return;
+  }
+
+  wrenSetSlotBool(vm, 0, true);
+  wrenFreeVM(otherVM);
+}
+
+WrenForeignMethodFn userDataBindMethod(const char* signature)
+{
+  if (strcmp(signature, "static UserData.test") == 0) return test;
+
+  return NULL;
+}