Consistent use of anonymous namespace

Objects that are only accessible at file-scope should be put in the anonymous namespace.
This is what the  C++ standard recommends, rather than using static, which is really C-style and results in static linkage.

Stockfish already does this throughout the code. So let's weed out the few exceptions,
because... they have no reason to be exceptional.

No functional change.

Resolves #84

src/movepick.cpp

@@ -61,7 +61,7 @@ namespace {
       std::swap(*begin, *std::max_element(begin, end));
       return begin;
   }
-}
+} // namespace
 
 
 /// Constructors of the MovePicker class. As arguments we pass information