These where simple to implement (almost too simple ... I probably
got something wrong!)

Tested powerpc64le-linux, committed to trunk.


(Apart from using common_type_t, which is easy to change, these
functions meet the simpler rules for C++11 constexpr, so moving them
out of <experimental/numeric> would probably allow <ratio> to be
greatly simplified. I don't plan on doing that myself any time soon,
but it would make sense to do it some day.)