This fixes issue #84.

This way, iSplit will work when one side is persistent.

This could lead to awkward loops, for example, when having:

- As goal `own γ c` with `c` persistent, one could keep on
  `iSplit`ting the goal. Especially in (semi-)automated proof
  scripts this is annoying as it easily leads to loops.
- When having a hypothesis `own γ c` with `c` persistent, one
  could keep on `iDestruct`ing it.

To that end, this commit removes the `IntoOp` and `FromOp` instances
for persistent CMRA elements. Instead, we changed the instances for
pairs, so that one, for example, can still split `(a ⋅ b, c)` with
`c` persistent.

This are useful as proofmode cannot always guess in which direction
it should use ⊣⊢.

This fixes issue #81.

Updating the ProofMode.md docs

See merge request !53

The instances frame_big_sepL_cons and frame_big_sepL_app could be
applied repeatedly often when framing in [∗ list] k ↦ x ∈ ?e, Φ k x
when ?e an evar. This commit fixes this bug.

- Allow framing of persistent hypotheses below the always modality.
- Allow framing of persistent hypotheses in just one branch of a
  disjunction.