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/blockchain.cpp

@@ -61,6 +61,8 @@
 
 #define FIND_BLOCKCHAIN_SUPPLEMENT_MAX_SIZE (100*1024*1024) // 100 MB
 
+using namespace crypto;
+
 //#include "serialization/json_archive.h"
 
 /* TODO: