Rename Position::list

Use Position::square and Position::squares instead.

This allow us to remove king_square(), simplify
endgames and to have more naming uniformity.

Moreover, this is a prerequisite step in case
in the future we decide to retire piece lists
altoghter and use pop_lsb() to loop across
pieces and serialize the moves. In this way
we just need to change definition of Position::square
to something like:

template<PieceType Pt> inline
Square Position::square(Color c) const {
  return lsb(byColorBB[c]);
}

No functional change.