Various improvements to the ZMQ JSON-RPC handling:

- Finding handling function in ZMQ JSON-RPC now uses binary search
  - Temporary `std::vector`s in JSON output now use `epee::span` to
    prevent allocations.
  - Binary -> hex in JSON output no longer allocates temporary buffer
  - C++ structs -> JSON skips intermediate DOM creation, and instead
    write directly to an output stream.

contrib/epee/include/hex.h

@@ -51,12 +51,22 @@ namespace epee
     template<std::size_t N>
     static std::array<char, N * 2> array(const std::array<std::uint8_t, N>& src) noexcept
     {
-      std::array<char, N * 2> out{{}};
+      std::array<char, N * 2> out;
       static_assert(N <= 128, "keep the stack size down");
       buffer_unchecked(out.data(), {src.data(), src.size()});
       return out;
     }
 
+    //! \return An array containing hex of `src`.
+    template<typename T>
+    static std::array<char, sizeof(T) * 2> array(const T& src) noexcept
+    {
+      std::array<char, sizeof(T) * 2> out;
+      static_assert(sizeof(T) <= 128, "keep the stack size down");
+      buffer_unchecked(out.data(), as_byte_span(src));
+      return out;
+    }
+
     //! Append `src` as hex to `out`.
     static void buffer(std::ostream& out, const span<const std::uint8_t> src);