These indices have existed in dune-typetree for a while.  They are already used extensively
in dune-typetree and dune-functions, and they also make working with MultiTypeBlockVector/Matrix
from dune-istl much nicer.  They work very much like std::placeholders:_1, std::placeholders::_2,
etc.  We would have like to use that, but there is no std::placeholders_0.

This patch also introduces

template<std::size_t i>
using index_constant = std::integral_constant<std::size_t, i>;

which helps decrease code verbosity a little bit.