Introduce namespace Pawns

And retire old struct PawnTable along the same lines
of previous patch.

No functional change.

src/thread.h

@@ -112,7 +112,7 @@ public:
   Eval::Table evalTable;
   Material::Table materialTable;
   Endgames endgames;
-  PawnTable pawnTable;
+  Pawns::Table pawnsTable;
   size_t idx;
   int maxPly;
   Mutex mutex;