[thread.mutex] Mutex types defined in requirements

[K-ballo]

The mutex types are defined together with their requirements, under the "Mutex Requirements" [thread.mutex.requirements] section. This feels a bit odd.

I was expecting only the requirements to be defined under "Mutex Requirements", and the actual classes to be defined in their own sections, at the same level of requirements. See for example how the [containers] clause is specified.

[jensmaurer]

We could certainly move the subclauses containing the class definitions directly under [thread.mutex]. I'm not sure it would be a net improvement.

[zygoloid]

It seems like we have mixed together two things here: a set of requirements on mutex types, and a description of some concrete types that satisfy those requirements. We should describe the requirements abstractly, and then say that the standard library mutex types meet them, as we do with containers etc.

This is a large reorganization, and I'd like to request that LWG find someone to pursue it rather than us doing it editorially.

[jwakely]

I'll write a P-paper proposing that reorg.

[jwakely]

Unassigning myself, as I don't plan to work on this now.