Use c++ in runtime src file

It makes string manipulation easier

src/runtime/tree.cpp

@@ -0,0 +1,55 @@
+#include "tree_sitter/runtime.h"
+#include <string>
+#include <string.h>
+
+using std::string;
+
+TSTree * TSTreeMake(TSSymbol value, size_t child_count, TSTree **children) {
+    TSTree *result = new TSTree();
+    result->value = value;
+    result->child_count = child_count;
+    result->children = children;
+    result->ref_count = 0;
+    for (int i = 0; i < child_count; i++)
+        TSTreeRetain(children[i]);
+    return result;
+}
+
+void TSTreeRetain(TSTree *tree) {
+    tree->ref_count++;
+}
+
+void TSTreeRelease(TSTree *tree) {
+    tree->ref_count--;
+    if (tree->ref_count == 0) {
+        for (int i = 0; i < tree->child_count; i++)
+            TSTreeRelease(tree->children[i]);
+        free(tree);
+    }
+}
+
+int TSTreeEquals(const TSTree *node1, const TSTree *node2) {
+    if (node1->value != node2->value) return 0;
+    if (node1->child_count != node2->child_count) return 0;
+    for (int i = 0; i < node1->child_count; i++) {
+        TSTree *child1 = node1->children[i];
+        TSTree *child2 = node2->children[i];
+        if (!TSTreeEquals(child1, child2)) return 0;
+    }
+    return 1;
+}
+
+static string __tree_to_string(const TSTree *tree, const char **symbol_names) {
+    if (!tree) return "#<null-tree>";
+    string result = string("(") + symbol_names[tree->value];
+    for (int i = 0; i < tree->child_count; i++)
+        result += " " + __tree_to_string(tree->children[i], symbol_names);
+    return result + ")";
+}
+
+char * TSTreeToString(const TSTree *tree, const char **symbol_names) {
+    string value(__tree_to_string(tree, symbol_names));
+    char *result = (char *)malloc(value.size());
+    strcpy(result, value.c_str());
+    return result;
+}