Asymmetry bonus for the attacking side

Use asymmetry in the position (king separation, pawn structure) to
compute an "initiative bonus" for the attacking side.

Passed STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 14563 W: 2826 L: 2636 D: 9101

And LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 14363 W: 2317 L: 2141 D: 9905

Bench: 8116244

Resolves #462

src/pawns.cpp

@@ -243,6 +243,7 @@ Entry* probe(const Position& pos) {
 
   e->key = key;
   e->score = evaluate<WHITE>(pos, e) - evaluate<BLACK>(pos, e);
+  e->asymmetry = popcount<Max15>( e->semiopenFiles[WHITE] ^ e->semiopenFiles[BLACK] );
   return e;
 }