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/wallet/wallet2.cpp

@@ -67,6 +67,8 @@ extern "C"
 #include "crypto/keccak.h"
 #include "crypto/crypto-ops.h"
 }
+using namespace std;
+using namespace crypto;
 using namespace cryptonote;
 
 #undef MONERO_DEFAULT_LOG_CATEGORY