Add unit test for stack

- Also, fix bug where trees pushed onto the stack were not retained

spec/runtime/helpers/tree_helpers.cc

@@ -0,0 +1,8 @@
+#include "helpers/tree_helpers.h"
+
+ts_tree ** tree_array(std::vector<ts_tree *> trees) {
+    ts_tree ** result = (ts_tree **)calloc(trees.size(), sizeof(ts_tree *));
+    for (size_t i = 0; i < trees.size(); i++)
+        result[i] = trees[i];
+    return result;
+}

spec/runtime/helpers/tree_helpers.h

@@ -0,0 +1,5 @@
+#include "tree_sitter/runtime.h"
+
+#include <vector>
+
+ts_tree ** tree_array(std::vector<ts_tree *> trees);