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/include/hex.h

@@ -0,0 +1,78 @@
+// Copyright (c) 2017, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// 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.
+
+#pragma once
+
+#include <array>
+#include <cstdint>
+#include <iosfwd>
+#include <string>
+
+#include "span_view.h"
+
+namespace epee
+{
+  struct to_hex
+  {
+    //! \return A std::string containing hex of `src`.
+    static std::string string(const view<std::uint8_t> src);
+
+    //! \return A std::string containing hex of `src`.
+    static std::string string(const view<char> src) {
+      return string(view_cast<std::uint8_t>(src));
+    }
+
+    //! \return An array containing hex of `src`.
+    template<std::size_t N>
+    static std::array<char, N * 2>
+    array(const std::array<std::uint8_t, N>& src) noexcept(noexcept(view<std::uint8_t>(src)))
+    {
+      std::array<char, N * 2> out{{}};
+      static_assert(N <= 128, "keep the stack size down");
+      buffer_unchecked(out.data(), src);
+      return out;
+    }
+
+    //! Append `src` as hex to `out`.
+    static void buffer(std::ostream& out, const view<std::uint8_t> src);
+
+    //! Append `< + src + >` as hex to `out`.
+    static void formatted(std::ostream& out, const view<std::uint8_t> src);
+
+    //! Append `< + src + >` as hex to `out`.
+    template<typename T>
+    static void formatted_from_pod(std::ostream& out, const T& pod)
+    {
+      formatted(out, pod_cast<const std::uint8_t>(pod));
+    }
+
+  private:
+    //! Write `src` bytes as hex to `out`. `out` must be twice the length
+    static void buffer_unchecked(char* out, const view<std::uint8_t> src) noexcept;
+  };
+}