[allocator.requirements.general] Add namespace std for exposition-only concept

[hewillk]

No description provided.

[jwakely]

We don't do this elsewhere for exposition-only helpers, do we?

e.g. [container.intro.reqmts] and [sequences.general] and [associative.general].

Is there any possible confusion about what this exposition-only concept means? Is it relevant which namespace it's a member of, given that users can never refer to it?

[hewillk]

Maybe. But this is only to consistent with [expos.only.entity] which also belongs to [library].

[tkoeppe]

Hm, it's true that we put the expos-only entities in [library] in namespace std. It's only in the specific library facilities where locally used expos-only entities are declared without namespace.

Ultimately I don't think it matters for the normative effect, but I can see an consistency argument for the present case.

[jwakely]

Yeah, i don't mind either way.