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`

tests/unit_tests/ringct.cpp

@@ -32,6 +32,7 @@
 
 #include <cstdint>
 #include <algorithm>
+#include <sstream>
 
 #include "ringct/rctTypes.h"
 #include "ringct/rctSigs.h"
@@ -1048,3 +1049,13 @@ TEST(ringct, reject_gen_non_simple_ver_simple)
   rct::rctSig sig = make_sample_rct_sig(NELTS(inputs), inputs, NELTS(outputs), outputs, true);
   ASSERT_FALSE(rct::verRctSimple(sig));
 }
+
+TEST(ringct, key_ostream)
+{
+  std::stringstream out;
+  out << "BEGIN" << rct::H << "END";
+  EXPECT_EQ(
+    std::string{"BEGIN<8b655970153799af2aeadc9ff1add0ea6c7251d54154cfa92c173a0dd39c1f94>END"},
+    out.str()
+  );
+}