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

ruby_parser.c

@@ -964,6 +964,13 @@ rb_parser_set_yydebug(VALUE vparser, VALUE flag)
 }
 #endif
 
+VALUE
+rb_node_sym_string_val(const NODE *node)
+{
+    rb_parser_string_t *str = RNODE_SYM(node)->string;
+    return ID2SYM(rb_intern3(str->ptr, str->len, str->enc));
+}
+
 VALUE
 rb_node_line_lineno_val(const NODE *node)
 {