The traits class IntegerSequenceEntry computes the i-th entry
of an std::integer_sequence. The integerSequenceEntry helper
function does the same with function syntax.

This allows to avoid the hight template instantiation depth
of std::make_tuple when using the "natural" implementation

  std::get<index>(std::make_tuple(t...))

The letter adds 15 instantiation levels per argument whereas
the still recursive implementation in this commit adds only one.