[boost] [mpl] Proceeding with modularization

Hi, Now that 1.56 is out I believe we can proceed with modularization. I'd like to complete Boost.MPL decomposition into Boost.MPL.Core and Boost.MPL that I started some time ago. I can see two ways of doing this: I can keep the two parts in the separate git repositories or I can move Boost.MPL.Core to a sub- library within the same git repo. I'd prefer the second approach since this allows to create a single pull request and keeps the library history in one place, but maybe there are other considerations?