inverse function creating map<T, size_t>

[Peter]

There are two inverse functions in stl_utility. One creates std::multimap<T, size_t> and the other creates std::map<T, std::vector<size_t> >. Often I know that elements are identical I I want a map<T, size>. The inverse function could perhaps throw if elements are not unique or just leave the behaviour undefined.

[Peter]

(In [2857]) closes #726