Add tasks for all the new languages

Task/Tree-traversal/Sidef/tree-traversal.sidef

@@ -0,0 +1,28 @@
+func preorder(t) {
+    t ? [t[0], __FUNC__(t[1])..., __FUNC__(t[2])...] : [];
+}
+
+func inorder(t) {
+    t ? [__FUNC__(t[1])..., t[0], __FUNC__(t[2])...] : [];
+}
+
+func postorder(t) {
+    t ? [__FUNC__(t[1])..., __FUNC__(t[2])..., t[0]] : [];
+}
+
+func depth(t) {
+    var a = [t];
+    var ret = [];
+    while (a.len > 0) {
+        var v = (a.shift \\ next);
+        ret « v[0];
+        a += [v[1,2]];
+    };
+    return ret;
+}
+
+var x = [1,[2,[4,[7]],[5]],[3,[6,[8],[9]]]];
+say "pre:   #{preorder(x)}";
+say "in:    #{inorder(x)}";
+say "post:  #{postorder(x)}";
+say "depth: #{depth(x)}";