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`

contrib/epee/src/CMakeLists.txt

@@ -26,7 +26,7 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-add_library(epee STATIC http_auth.cpp mlog.cpp string_tools.cpp)
+add_library(epee STATIC hex.cpp http_auth.cpp mlog.cpp string_tools.cpp)
 # Build and install libepee if we're building for GUI
 if (BUILD_GUI_DEPS)
     if(IOS)