Rename FromTo -> History (#963)

Previously, we had duplicated History:

- one with (piece,to) called History
- one with (from,to) called FromTo

Now that we have only one, rename it to History, which is the generally accepted
name in the chess programming litterature for this technique.

Also correct some comments that had not been updated since the introduction of CMH.

No functional change.

src/thread.h

@@ -70,7 +70,7 @@ public:
   Depth completedDepth;
   std::atomic_bool resetCalls;
   MoveStats counterMoves;
-  FromToStats fromTo;
+  HistoryStats history;
   CounterMoveHistoryStats counterMoveHistory;
 };