Add day 7

EnumerableExtensions.cs

@@ -0,0 +1,20 @@
+namespace AdventOfCode;
+
+public static class EnumerableExtensions
+{
+    public static IEnumerable<TSource> Flatten<TSource>(this TSource root, Func<TSource, IEnumerable<TSource>> flattener)
+    {
+        var toVisit = new Stack<TSource>();
+        toVisit.Push(root);
+
+        while (toVisit.TryPop(out var node))
+        {
+            yield return node;
+        
+            foreach (var child in flattener(node))
+            {
+                toVisit.Push(child);
+            }
+        }
+    }
+}