Variant-specific endgame code

[ianfab]

For built-in variants support variant-specific endgame code in order to collect variant endgame knowledge in one place and to improve playing strength. Examples:

• Makruk https://github.com/ianfab/Makruk-Stockfish/blob/da3b89d87c21b9bfc2ea38927e2b22e8b336f926/src/endgame.cpp#L89-L104
• Shatranj https://github.com/ianfab/Shatranj-Stockfish/blob/218ee0a6dec87cf77e09a30947700fc794fd3d5d/src/endgame.cpp#L80-L90
• Atomic https://github.com/ddugovic/Stockfish/blob/8a59ec9844987ca493106465de9029b6ec59b574/src/endgame.cpp#L97-L102
• Antichess https://github.com/ddugovic/Stockfish/blob/8a59ec9844987ca493106465de9029b6ec59b574/src/endgame.cpp#L92-L94
• S-Chess https://github.com/ianfab/Seirawan-Stockfish/blob/dad6e2154ba48f2ff9427fc3923ea2282b74f9e1/src/endgame.cpp#L101-L102
• Misere

  Fairy-Stockfish/src/endgame.cpp

  Lines 147 to 178 in c322875

  	/// Self-mate with KX vs KX.
  	template<>
  	Value Endgame<KXKX>::operator()(const Position& pos) const {
  	assert(!pos.checkers()); // Eval is never called when in check
  	// Stalemate detection with lone king
  	if (pos.side_to_move() == weakSide && !MoveList<LEGAL>(pos).size())
  	return VALUE_DRAW;
  	Square strongKing = pos.square<KING>(strongSide);
  	Square weakKing = pos.square<KING>(weakSide);
  	Value result = pos.non_pawn_material(strongSide) * int(VALUE_KNOWN_WIN) / int(VALUE_KNOWN_WIN + pos.non_pawn_material(strongSide))
  	- pos.non_pawn_material(weakSide)
  	+ pos.count<PAWN>(weakSide) * PawnValueEg
  	+ push_to_opposing_edge(relative_square(weakSide, strongKing, pos.max_rank()), pos) * 2
  	+ push_close(strongKing, weakKing) * 2;
  	for (Bitboard b = pos.pieces(PAWN); b;)
  	{
  	Square s = pop_lsb(b);
  	result += (push_close(strongKing, s) + push_close(weakKing, s)) / 2;
  	}
  	if (!pos.count<PAWN>(weakSide))
  	result = VALUE_DRAW;
  	else if (pos.count<PAWN>(weakSide) == 1)
  	result = result / 2;
  	return strongSide == pos.side_to_move() ? result : -result;
  	}