Recover `String#to_str`; ref #4177

We have removed implicit conversion to strings using `to_str`. But some
people still using `to_str` as a typical string method, i.e. they do
string check by code like: `obj.respond_to?(:to_str)`. So we have
recovered the method.

src/string.c

@@ -2754,6 +2754,7 @@ mrb_init_string(mrb_state *mrb)
 #endif
   mrb_define_method(mrb, s, "to_i",            mrb_str_to_i,            MRB_ARGS_ANY());  /* 15.2.10.5.39 */
   mrb_define_method(mrb, s, "to_s",            mrb_str_to_s,            MRB_ARGS_NONE()); /* 15.2.10.5.40 */
+  mrb_define_method(mrb, s, "to_str",          mrb_str_to_s,            MRB_ARGS_NONE());
   mrb_define_method(mrb, s, "to_sym",          mrb_str_intern,          MRB_ARGS_NONE()); /* 15.2.10.5.41 */
   mrb_define_method(mrb, s, "upcase",          mrb_str_upcase,          MRB_ARGS_NONE()); /* 15.2.10.5.42 */
   mrb_define_method(mrb, s, "upcase!",         mrb_str_upcase_bang,     MRB_ARGS_NONE()); /* 15.2.10.5.43 */