Introduce NODE_SYM to manage symbol literal

`:sym` was managed by `NODE_LIT` with `Symbol` object.
This commit introduces `NODE_SYM` so that

1. Symbol literal is detectable from AST Node
2. Reduce dependency on ruby object

node.c

@@ -179,6 +179,9 @@ static void
 free_ast_value(rb_ast_t *ast, void *ctx, NODE *node)
 {
     switch (nd_type(node)) {
+      case NODE_SYM:
+        parser_string_free(ast, RNODE_SYM(node)->string);
+        break;
       case NODE_FILE:
         parser_string_free(ast, RNODE_FILE(node)->path);
         break;