add mrb_string_type().

include/mruby/string.h

@@ -41,6 +41,7 @@ mrb_value mrb_ptr_to_str(mrb_state *, void *);
 mrb_value mrb_obj_as_string(mrb_state *mrb, mrb_value obj);
 mrb_value mrb_str_resize(mrb_state *mrb, mrb_value str, mrb_int len);
 mrb_value mrb_str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len);
+mrb_value mrb_string_type(mrb_state *mrb, mrb_value str);
 mrb_value mrb_check_string_type(mrb_state *mrb, mrb_value str);
 mrb_value mrb_str_buf_new(mrb_state *mrb, mrb_int capa);
 mrb_value mrb_str_buf_cat(mrb_state *mrb, mrb_value str, const char *ptr, size_t len);

src/string.c

@@ -1523,6 +1523,12 @@ mrb_ptr_to_str(mrb_state *mrb, void *p)
   return mrb_obj_value(p_str);
 }
 
+mrb_value
+mrb_string_type(mrb_state *mrb, mrb_value str)
+{
+  return mrb_convert_type(mrb, str, MRB_TT_STRING, "String", "to_str");
+}
+
 mrb_value
 mrb_check_string_type(mrb_state *mrb, mrb_value str)
 {