Improvements for epee binary to hex functions:

- Performance improvements
  - Added `span` for zero-copy pointer+length arguments
  - Added `std::ostream` overload for direct writing to output buffers
  - Removal of unused `string_tools::buff_to_hex`

src/cryptonote_basic/cryptonote_basic.h

@@ -44,7 +44,6 @@
 #include "serialization/debug_archive.h"
 #include "serialization/crypto.h"
 #include "serialization/keyvalue_serialization.h" // eepe named serialization
-#include "string_tools.h"
 #include "cryptonote_config.h"
 #include "crypto/crypto.h"
 #include "crypto/hash.h"