feat(data/nat/digits): digits_len (#11187)
feat(data/nat/digits): digits_len (#11187)

Via a new `data.nat.log` import.
Also unprivate `digits_eq_cons_digits_div`.

The file needs a refactor to make the names more mathlib-like,
otherwise I would have named it `length_digits`.






leanprover-community/mathlib,leanprover-community/mathlib