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/ringct.cpp

@@ -38,6 +38,7 @@
 #include "ringct/rctSigs.h"
 #include "ringct/rctOps.h"
 
+using namespace std;
 using namespace crypto;
 using namespace rct;