Assorted code style and comments in pawns.cpp and pawns.h

The only interesting thing is that a backward or isolated
pawn cannot be a candidate passer, so code this condition.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/tt.h

@@ -51,6 +51,7 @@ public:
   virtual ~SimpleHash() { delete [] entries; }
 
   Entry* find(Key key) const { return entries + ((uint32_t)key & (HashSize - 1)); }
+  void prefetch(Key key) const { ::prefetch((char*)find(key)); }
 
 protected:
   Entry* entries;