Add Stockfish namespace.

fixes #3350 and is a small cleanup that might make it easier to use SF
in separate projects, like a NNUE trainer or similar.

closes https://github.com/official-stockfish/Stockfish/pull/3370

No functional change.

src/types.h

@@ -83,6 +83,8 @@
 #  define pext(b, m) 0
 #endif
 
+namespace Stockfish {
+
 #ifdef USE_POPCNT
 constexpr bool HasPopCnt = true;
 #else
@@ -482,6 +484,8 @@ constexpr Key make_key(uint64_t seed) {
   return seed * 6364136223846793005ULL + 1442695040888963407ULL;
 }
 
+} // namespace Stockfish
+
 #endif // #ifndef TYPES_H_INCLUDED
 
 #include "tune.h" // Global visibility to tuning setup