Stop using Arena type since everything can be GC'd easily

Solver/Crafty/ArenaNode.cs

@@ -0,0 +1,25 @@
+using System.Runtime.CompilerServices;
+
+namespace Craftimizer.Solver.Crafty;
+
+public class ArenaNode<T> where T : struct
+{
+    public readonly T State;
+    public readonly List<ArenaNode<T>> Children;
+    public readonly ArenaNode<T>? Parent;
+
+    public ArenaNode(T state, ArenaNode<T>? parent = null)
+    {
+        State = state;
+        Children = new();
+        Parent = parent;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public ArenaNode<T> Add(T state)
+    {
+        var node = new ArenaNode<T>(state, this);
+        Children.Add(node);
+        return node;
+    }
+}