wallet: wipe seed from memory where appropriate

contrib/epee/include/hex.h

@@ -33,6 +33,7 @@
 #include <iosfwd>
 #include <string>
 
+#include "wipeable_string.h"
 #include "span.h"
 
 namespace epee
@@ -41,6 +42,8 @@ namespace epee
   {
     //! \return A std::string containing hex of `src`.
     static std::string string(const span<const std::uint8_t> src);
+    //! \return A epee::wipeable_string containing hex of `src`.
+    static epee::wipeable_string wipeable_string(const span<const std::uint8_t> src);
 
     //! \return An array containing hex of `src`.
     template<std::size_t N>
@@ -59,6 +62,8 @@ namespace epee
     static void formatted(std::ostream& out, const span<const std::uint8_t> src);
 
   private:
+    template<typename T> T static convert(const span<const std::uint8_t> src);
+
     //! Write `src` bytes as hex to `out`. `out` must be twice the length
     static void buffer_unchecked(char* out, const span<const std::uint8_t> src) noexcept;
   };