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;

src/cryptonote_core/cryptonote_tx_utils.cpp

@@ -40,6 +40,8 @@ using namespace epee;
 #include "crypto/hash.h"
 #include "ringct/rctSigs.h"
 
+using namespace crypto;
+
 namespace cryptonote
 {
   //---------------------------------------------------------------