Remove using namespace std;

For historical reasons we had a "using namespace std;" in port.h. This
is generally a bad idea. So removing it and along the way doing a bunch
of cpplint cleanup.

Change-Id: Ia125601a55ae62695e247fb0250df4c6f86c46c6

internal/ceres/array_utils.h

@@ -43,6 +43,7 @@
 #ifndef CERES_INTERNAL_ARRAY_UTILS_H_
 #define CERES_INTERNAL_ARRAY_UTILS_H_
 
+#include <string>
 #include "ceres/internal/port.h"
 
 namespace ceres {