No longer allow ownership of persistent elements to be split.

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.