Lots of windows fixes and some more improvements.

source/mijin/debug/stacktrace.cpp

@@ -100,7 +100,7 @@ Result<Stacktrace> captureStacktrace(unsigned skipFrames) MIJIN_NOEXCEPT
     return Stacktrace(std::move(btData.stackframes));
 #else // MIJIN_USE_LIBBACKTRACE
 	(void) skipFrames;
-	return {}; // TODO
+	return ResultError("not implemented");
 #endif // MIJIN_USE_LIBBACKTRACE
 }