boost::mpl::sequence_tag conflicts with mp11's one

[redboltz]

It seems that
https://github.com/boostorg/fusion/blob/develop/include/boost/fusion/adapted/std_tuple/tag_of.hpp#L32
conflicts with
https://github.com/boostorg/mp11/blob/develop/include/boost/mp11/mpl.hpp#L148

Running demo:
https://wandbox.org/permlink/BoAQGjmPMWTW0Uwj

See boostorg/fusion#182 (comment)

I'm not sure which library should define sequence_tag. Or some it should be defined in the common header?

[pdimov]

In 164aa37, I've split mpl.hpp into mpl_list.hpp and mpl_tuple.hpp, so in case of a conflict with Fusion libraries can now include just mpl_list.hpp and leave std::tuple to Fusion.