refactor(*iterate*): move to `function`; renamings (#2832)
* move lemmas about `iterate` from `data.nat.basic` to `logic.function.iterate`;
* move lemmas like `nat.iterate_succ` to `function` namespace;
* rename some lemmas:
- `iterate₀` to `iterate_fixed`,
- `iterate₁` to `semiconj.iterate_right`, see also `commute.iterate_left` and `commute.iterate_right`;
- `iterate₂` to `semiconj₂.iterate`;
- `iterate_cancel` to `left_inverse.iterate` and `right_inverse.iterate`;
* move lemmas `*_hom.iterate_map_*` to `algebra/iterate_hom`.
