Implement workarounds for broken sizeof...

This adds Dune::SizeOf that can be use as Dune::SizeOf<T...>::value as a drop in replacement for sizeof...(T). This is necessary because the latter is broken in clang<3.8. Because the bug also effects std::index_sequence_for we now always provide our manually written fallback implementation Std::index_sequence_for that works around the bug by using SizeOf.

As a rule of thumb you should always use SizeOf instead instead of sizeof... in variadic template aliases if you want to avoid compiling code that produces wrong results.