remove "using namespace std" from headers

It's nasty, and actually breaks on Solaris, where if.h fails to
build due to:

  struct map *if_memmap;

tests/unit_tests/serialization.cpp

@@ -49,6 +49,7 @@
 #include "gtest/gtest.h"
 #include "unit_tests_utils.h"
 using namespace std;
+using namespace crypto;
 
 struct Struct
 {