Corrade/Utility/StlForwardTupleSizeElement.h file new in Git master

Forward declaration for std::tuple_size and std::tuple_element

On libc++, libstdc++ and MSVC STL forward-declares the std::tuple_size and std::tuple_element types inside the STL namespace in an implementation-specific way. On other implementations is equivalent to #include <utility>, which is guaranteed to contain those in order to define them for std::pair.