Single header version of mp11

It would be great to have boost.mp11 available as a single header with root namespace that can be defined from outside. This allows to distribute mp11 along with a library that needs MPL but does not otherwise depend on Boost. Having a definable namespace allows each separate library to distribute its own copy of mp11 without conflicts. For example I’d like to use mp11 in my [Ppconsul](https://github.com/oliora/ppconsul) project without introducing a dependency on *modern* Boost (one is not available on some platforms where Ppconsul is used). Currently Ppconsul uses Boost.MPL and Boost.Fusion which I want to get rid of in favor of some modern MPL library as formers are slow to compile and harder to use.

Other modern MPL libraries usually have a single header version ( I guess for exact same reason).

If there is a general interest in getting a single header mp11 I can implement it.