Recover `String#to_str`; ref
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.
MODIFY
src/string.c
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 */

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.