Restore `mrb_string_type` function for compatibility.

include/mruby/string.h

@@ -313,6 +313,10 @@ MRB_API mrb_value mrb_str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb
  */
 MRB_API mrb_value mrb_ensure_string_type(mrb_state *mrb, mrb_value str);
 MRB_API mrb_value mrb_check_string_type(mrb_state *mrb, mrb_value str);
+/* obsolete: use mrb_ensure_string_type() instead */
+MRB_API mrb_value mrb_string_type(mrb_state *mrb, mrb_value str);
+
+
 MRB_API mrb_value mrb_str_new_capa(mrb_state *mrb, size_t capa);
 MRB_API mrb_value mrb_str_buf_new(mrb_state *mrb, size_t capa);
 

src/object.c

@@ -589,6 +589,13 @@ mrb_to_str(mrb_state *mrb, mrb_value val)
   return val;
 }
 
+/* obsolete: use mrb_ensure_string_type() instead */
+MRB_API mrb_value
+mrb_string_type(mrb_state *mrb, mrb_value str)
+{
+  return mrb_ensure_string_type(mrb, str);
+}
+
 MRB_API mrb_value
 mrb_ensure_string_type(mrb_state *mrb, mrb_value str)
 {