mathlib3

fc2e3b00 - refactor(algebra/free_monoid): add `to_list`/`of_list` (#16926)

refactor(algebra/free_monoid): add `to_list`/`of_list` (#16926) * add `free_monoid.to_list` and `free_monoid.of_list`; * update API to explicitly use these functions; * fix `control.fold`.