chore(algebra/group_with_zero_power): review (#2966)
List of changes:
* Rename `gpow_neg_succ` to `gpow_neg_succ_of_nat` to match other names in `int` namespace.
* Add `units.coe_gpow`.
* Remove `fpow_neg_succ`, leave `fpow_neg_succ_of_nat`.
* Rewrite the proof of `fpow_add` in the same way I rewrote the proof of `gpow_add`.
* Make argument `a` implicit in some lemmas because they have an argument `ha : a ≠ 0`.
* Remove `fpow_inv`. This was a copy of `fpow_neg_one` with a misleading name.
* Remove `unit_pow` in favor of a more general `units.coe_pow`.
* Remove `unit_gpow`, add a more general `units.coe_gpow'` instead.
