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This is an alternative to !224.

This allows one to make use of recursive ghost state obtained from the
recursive domain equation solver.

We never make canonical instances of them.

With Coq master, we otherwise fail to infer instances with subG_savedAnythingΣ, which leads to goals like

> looking for (subG
>                       (@savedAnythingΣ (@ofe_funCF val (fun _ : val => laterCF idCF))
>                          ?cFunctorContractive0) Σ)

This is a follow up of !248.

Also fixes pre-existing bug in iCombine error messages.

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.