Result: Add try_set_value()

This helper returns true if the Result contains a value, or false if it doesn't.
Additionally, if it has a value, it sets the passed reference to it.

luna/Result.h

@@ -104,6 +104,13 @@ template <typename T> class Result
         return other;
     }
 
+    bool try_set_value(T& ref)
+    {
+        if (!has_value()) return false;
+        ref = m_storage.fetch_reference();
+        return true;
+    }
+
     T release_value()
     {
         check(has_value());