Should we move detail_:: code and/or implementations to separate files?

[eyalroz]

A lot of the wrapper code is located by now within detail:: sub-namespaces, interspersed among the actual, intended-for-use, functions. Additionally, a lot of the implementations of non-detail:: functions are already separated from the declarations, because they require the definitions of wrapper classes which the function declaration does not depend on (multi_header_impls.hpp).

We could, in theory:

1. Move all detail:: code into separate files within a detail subdirectory (with careful planning to work around dependencies with the non-detail file, e.g. type definitions.
2. Move all implementations to different files (breaking up multi_wrapper_impls.hpp.

But I'm not sure this is all worth it.