Rename `detail` namespace to `auimpl`

[chiphogg]

Motivation: https://abseil.io/tips/130.

Try to give sub-namespaces unique names that are unlikely to collide with future top-levels.

[hoffbrinkle]

Note that our style guide has:

... within a namespace that includes the word internal...

Abseil looks like it uses this convention as well, further splitting up their internal namespaces, so magnitude_internal, quantity_internal, etc.).

[chiphogg]

That's a good point, and maybe it's what we'd do if we were starting over. That said, I think we're small enough in scope to get away with one "detail" namespace. And I picked auimpl because (a) it's very unlikely to ever be used by anyone else at all, and (b) it's the same length as detail, which should minimize churn in the PR.

[chiphogg]

Note that the first step will be to give people a migration path such that they're not specializing detail::UnitAvoidance. We need to remove this public-facing usage of the detail namespace. I'll try to come up with a plan for that.