More consistent naming for `auth`.
This MR is a follow up on the renamings performed (implicitly) as part of !215. This MR makes the following changes: - `auth_both_frac_valid` and `auth_both_valid` are now of the same shape as `auth_both_frac_validN` and `auth_both_validN`. That is, both are now biimplications. - The left-to-right direction of `auth_both_frac_valid` and `auth_both_valid` only holds in case the camera is discrete. The right-to-left versions for non-discrete cameras are prefixed `_2`, the convention that we use throughout the development. - Change the direction of lemmas like `auth_frag_valid` and `auth_auth_valid` so that it's consistent with the other lemmas. I.e. make sure that the ◯ and ● are always on the LHS of the biimplication.