rename Z2Nat_inj_div and Z2Nat_inj_mod

This MR renames Z2Nat_inj_div, Z2Nat_inj_mod, Nat2Z_inj_div and Nat2Z_inj_mod to follow the naming conventions of Z2Nat and Nat2Z. See !136 (comment 49866)

Should I mention iris-users here already or only if the MR is merged?