refactor(*): rename fpow and gpow to zpow (#9989)
Historically, we had two notions of integer power, one on groups called `gpow` and the other one on fields called `fpow`. Since the introduction of `div_inv_monoid` and `group_with_zero`, these definitions have been merged into one, and the boundaries are not clear any more. We rename both of them to `zpow`, adding a subscript `0` to disambiguate lemma names when there is a collision, where the subscripted version is for groups with zeroes (as is already done for inv lemmas).
To limit the scope of the change. this PR does not rename `gsmul` to `zsmul` or `gmultiples` to `zmultiples`.
