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.

src/pawns.cpp

@@ -110,7 +110,7 @@ namespace {
     Square s;
     bool passed, isolated, opposed, backward, lever, connected;
     Score score = SCORE_ZERO;
-    const Square* pl = pos.list<PAWN>(Us);
+    const Square* pl = pos.squares<PAWN>(Us);
     const Bitboard* pawnAttacksBB = StepAttacksBB[make_piece(Us, PAWN)];
 
     Bitboard ourPawns   = pos.pieces(Us  , PAWN);