estd: add index_sequence stuff from C++14
estd: add index_sequence stuff from C++14

Ableton/atria,respu/atria,dodheim/atria,bowlofstew/atria,manashmndl/atria,shaunstanislaus/atria,bowlofstew/atria,AbletonAG/atria,respu/atria,dodheim/atria,bowlofstew/atria,Ableton/atria,shaunstanislaus/atria,kod3r/atria,AbletonAG/atria,lyrl/atria,manashmndl/atria,lyrl/atria,iankronquist/atria,respu/atria,kod3r/atria,dodheim/atria,shaunstanislaus/atria,iankronquist/atria,lyrl/atria,kod3r/atria,Ableton/atria,iankronquist/atria,AbletonAG/atria,manashmndl/atria