uint256: replace sprintf with HexStr and reverse-iterator

Instead of calling sprintf for every byte, format the hex bytes
ourselves by help of HexStr and a reverse_iterator.

src/uint256.cpp

@@ -20,10 +20,7 @@ base_blob<BITS>::base_blob(const std::vector<unsigned char>& vch)
 template <unsigned int BITS>
 std::string base_blob<BITS>::GetHex() const
 {
-    char psz[sizeof(data) * 2 + 1];
-    for (unsigned int i = 0; i < sizeof(data); i++)
-        sprintf(psz + i * 2, "%02x", data[sizeof(data) - i - 1]);
-    return std::string(psz, psz + sizeof(data) * 2);
+    return HexStr(std::reverse_iterator<const uint8_t*>(data + sizeof(data)), std::reverse_iterator<const uint8_t*>(data));
 }
 
 template <unsigned int BITS>