Deprecate all _x procedures

[martinruefenacht]

Problem

The _x procedures are replaced by the _c procedures which were introduced through the Embiggenment. Deprecating the _x procedures allows for removal in the next major version.

Proposal

Deprecate all _x procedures in the Standard.

Changes to the Text

• MPI_Type_get_extent_x
• MPI_Type_get_true_extent_x
• MPI_Get_elements_x
• MPI_Type_size_x
• MPI_Status_set_elements_x

---

• Added text about deprecation of _x procedures in the Deprecated Interfaces chapter (16).
• Moving all _x procedure definitions to the Deprecated Interfaces chapter.
• Replacing references to _x procedures with _c procedures since they are equivalent.
• Add procedures to Table 2.1
• Add Changelog entry

Impact on Implementations

None, with deprecation compliant implementations still need to implement the procedures.

Impact on Users

Deprecation has no immediate effect, anyone who uses _x procedures is strongly encouraged to swap to the corresponding _c procedure with the identical signature.

References

Previous PR was: https://github.com/mpi-forum/mpi-standard/pull/623
Fixed by: https://github.com/mpi-forum/mpi-standard/pull/829

[Wee-Free-Scot]

@martinruefenacht is this going to be on the agenda for a reading at the Dec 22 meeting? There's one unresolved review comment (replacements exist for Fortran), but otherwise its ready. It would be a shame for this to miss the MPI-4.1 release.