Fix iPoseProof on recursive lemmas (fix #274)

When proving `foo` through a fixpoint, Coq's guardedness checker needs to see to
which arguments `foo` is applied. Opaque lemmas applied to `foo` itself prevent
that, so make them transparent.
* Make `IntoEmpValid` lemmas transparent.
* Expose application of `IntoEmpValid` instance to its argument.
* Add comment to `tac_pose_proof`

This MR brings back the type of `tac_pose_proof` to the one it had before !329.
Hence, this seems worth a comment.