Configurable top-level namespace (for standalone)

As you probably already know, MSVC uses Boost Math to implement the C++17 mathematical special functions. (My gratitude and joy over this fact cannot be expressed with words - I did _not_ want to spend a year of my life becoming expert enough in numerical analysis to implement these properly myself!) We have a thin wrapper layer that adapts the Standard's API to Boost's, which works great when folks link to the STL dynamically. We do have an issue, however, in that symbols in namespace `::boost` are directly visible in our static libraries, causing ODR problems for folks who are using different versions of Boost Math than we ship.

Would you be amenable to a PR that made the top-level namespace configurable if we were to implement such a feature? We could maintain a fork, of course, but (1) it seems like this might be useful for other folks that want to ship third-party libs using Boost Math, and (2) we like to keep the STL's list of dependencies short and clean to make it easier to onboard new contributors.