MPI requests are introduced too late

[devreal]

Problem

Prior to #586, requests were introduced in Section 3.7.1 (Communication Request Objects). Now, we use requests already for buffered operations introduced in Section 3.6 where it is entirely unclear what requests are and what to do with them.

Proposal

Requests are used in many parts of the standard (P2P, Collectives, RMA, IO) so they should be introduced as part of the semantic terms, with a forward reference to the completion functions. We already introduce MPI_Count in Section 2.5.8, so we could have a Section 2.5.9 for requests.

Impact on Implementations

None

Impact on Users

None, except that the text is better structured.

References and Pull Requests

TBD