Move Pieces[] out of global visibility

It is an helper array used only in position.cpp

Also small code tidy up while there.

No functional change.

Closes #1106

src/position.cpp

@@ -52,6 +52,9 @@ namespace {
 
 const string PieceToChar(" PNBRQK  pnbrqk");
 
+const Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
+                         B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
+
 // min_attacker() is a helper function used by see_ge() to locate the least
 // valuable attacker for the side to move, remove the attacker we just found
 // from the bitboards and scan for new X-ray attacks behind it.