Add C# alloc benchmark

benchmarks/csharp/Alloc.cs

@@ -0,0 +1,70 @@
+using Godot;
+
+public partial class Alloc : Benchmark
+{
+    const int ITERATIONS = 100_000;
+
+    void BenchmarkDeepTree() {
+        Node rt = new Node();
+        for (int i = 0; i < ITERATIONS; i++) {
+            Node n = new Node();
+            n.AddChild(rt);
+            rt = n;
+        }
+
+        // Avoid triggering a stack overflow with rt.free()
+        while (rt.GetChildCount() != 0) {
+            Node n = rt.GetChild(0);
+            rt.RemoveChild(n);
+            rt.Free();
+            rt = n;
+        }
+        rt.Free();
+    }
+
+    void BenchmarkWideTree() {
+        Node rt = new Node();
+        for (int i = 0; i < ITERATIONS; i++) {
+            rt.AddChild(new Node());
+        }
+        rt.Free();
+    }
+
+    void BenchmarkFragmentation() {
+        Node top = new Node();
+        for (int i = 0; i < 5; i++) {
+            top.AddChild(new Node());
+        }
+
+        for (int k = 0; k < 10; k++) {
+            for (int i = 0; i < ITERATIONS; i++) {
+                // Attempt to scatter children in memory by assigning newly created nodes to a random parent
+                int idx = (int)GD.Randi() % top.GetChildCount();
+                top.GetChild(idx).AddChild(new Node());
+            }
+
+            Node tmp = top.GetChild(0);
+            top.RemoveChild(tmp);
+            // Since nodes in the tree are scattered in memory,
+            // freeing subtrees this way should maximize fragmentation.
+            tmp.Free();
+            top.AddChild(new Node());
+            //GD.Print("Iteration %d: %.3f MB" % [k, Performance.get_monitor(Performance.MEMORY_STATIC) / 1e6])
+        }
+
+        top.Free();
+    }
+
+    void BenchmarkDuplicate() {
+        Node rt = new Node();
+        for (int i = 0; i < 16; i++) {
+            Node n = new Node();
+            n.AddChild(rt.Duplicate());
+            n.AddChild(rt.Duplicate());
+            rt.Free();
+            rt = n;
+            //GD.Print("Iteration %d: %.3f MB" % [i, Performance.get_monitor(Performance.MEMORY_STATIC) / 1e6])
+        }
+        rt.Free();
+    }
+}