Updated:
    src/metdata.hpp: Remove explicit "const" from key of std::map<>
       because the key is implicitly const already (see
       http://en.cppreference.com/w/cpp/container/map and
       http://www.cplusplus.com/reference/map/map/).

On some platforms (such as z/OS UNIX System Services) explicitly
declaring the map key as "const" causes template expansion errors
as it tries to create separate allocators for "const const std::string"
and "const std::string" only to find that they clash.  (Presumably
some compilers collapse these into one earlier.)

There are no template expansion errors if the map key is left to be
implicitly const.