Add bonus for advanced lever

Passed both STC
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 15413 W: 2670 L: 2530 D: 10213

And LTC:
LLR: 2.95 (-2.94,2.94) [1.00,6.00]
Total: 66908 W: 9398 L: 8960 D: 48550

Bench : 7859385

src/pawns.cpp

@@ -57,6 +57,11 @@ namespace {
     S( 0, 0), S( 6, 13), S(6,13), S(14,29),
     S(34,68), S(83,166), S(0, 0), S( 0, 0) };
 
+    // Levers bonus by rank
+  const Score Lever[RANK_NB] = {
+    S( 0, 0), S( 0, 0), S(0, 0), S(0, 0),
+    S(20,20), S(40,40), S(0, 0), S(0, 0) };
+
   // Bonus for file distance of the two outermost pawns
   const Score PawnsFileSpan = S(0, 15);
 
@@ -88,11 +93,12 @@ namespace {
     const Square Up    = (Us == WHITE ? DELTA_N  : DELTA_S);
     const Square Right = (Us == WHITE ? DELTA_NE : DELTA_SW);
     const Square Left  = (Us == WHITE ? DELTA_NW : DELTA_SE);
+    const Piece pc     = make_piece(Us, PAWN);
 
     Bitboard b, p, doubled;
     Square s;
     File f;
-    bool passed, isolated, opposed, connected, backward, candidate, unsupported;
+    bool passed, isolated, opposed, connected, backward, candidate, unsupported, lever;
     Score value = SCORE_ZERO;
     const Square* pl = pos.list<PAWN>(Us);
 
@@ -130,6 +136,7 @@ namespace {
         doubled     =   ourPawns   & forward_bb(Us, s);
         opposed     =   theirPawns & forward_bb(Us, s);
         passed      = !(theirPawns & passed_pawn_mask(Us, s));
+        lever       =   theirPawns & StepAttacksBB[pc][s];
 
         // Test for backward pawn.
         // If the pawn is passed, isolated, or connected it cannot be
@@ -192,6 +199,9 @@ namespace {
             if (!doubled)
                 e->candidatePawns[Us] |= s;
         }
+
+        if (lever)
+           value += Lever[relative_rank(Us, s)];
     }
 
     // In endgame it's better to have pawns on both wings. So give a bonus according