Follow-up from "Fix compilation with C++17"

The following discussion from !599 (merged) should be addressed:

• @clemenci started a discussion:

  @graven, in some other file you explicitly require as_const to be in the namespace details, which I find very unfortunate.

  I believe that when we backport features from future versions of the standard we should do it such that adopting the new standard less painful.

But putting it in in explicit namespace explicitly avoids clashes with the future standard... so depending on how you look at it, putting it in details could be considered 'less painful' as no changes should be required for the different standards.

mentioned in issue #237 (closed)