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_core.cpp

@@ -498,8 +498,8 @@ namespace cryptonote
       return false;
     }
 
-    tx_hash = null_hash;
-    tx_prefixt_hash = null_hash;
+    tx_hash = crypto::null_hash;
+    tx_prefixt_hash = crypto::null_hash;
 
     if(!parse_tx_from_blob(tx, tx_hash, tx_prefixt_hash, tx_blob))
     {