Store distinct upper and lower bound scores

This is more complex than what I'd like but I
was unable to split in small chunks.

Here we add 2 slots to TTEntry (valueUpper and depthUpper)
so that sizeof(TTEntry) returns to the original 16 bytes
and we can pack exactly 4 entries in a 64 bytes cache line.

Now we save an upper bound score alongside a lower (exact)
score. The idea is to increase TT cut-offs rates becuase
there is now an higher probability for a node to use TT info.

This patch is highly experimental and probably needs further
steps as is hinted by an unrealistic bench number:

bench: 2022385

src/types.h

@@ -163,7 +163,7 @@ enum Bound {
   BOUND_NONE  = 0,
   BOUND_UPPER = 1,
   BOUND_LOWER = 2,
-  BOUND_EXACT = BOUND_UPPER | BOUND_LOWER
+  BOUND_EXACT = BOUND_UPPER | BOUND_LOWER | 4
 };
 
 enum Value {