hide encoding helpers

lib/crypto/encoding.cpp

 #include <array>
+#include <string>
 #include <vector>
 
+namespace {
 template<std::size_t N, std::size_t... Is>
 constexpr std::array<char, N - 1>
 to_array(const char (&a)[N], std::index_sequence<Is...>)
@@ -163,6 +165,7 @@ decode_base64(const std::array<uint8_t, 256> &reverse_alphabet, const std::strin
 
         return decoded;
 }
+}
 
 namespace mtx {
 namespace crypto {