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.

Adding a hint without a database now triggers a deprecation warning in
Coq master (https://github.com/coq/coq/pull/8987).

This closes issue #220.