etl::expected doesn't compile with legacy_variant

[Un-done]

The expected class uses the variant constructor with 2 Arguments index and value

     ETL_CONSTEXPR14 expected() ETL_NOEXCEPT : storage(etl::in_place_index_t<Value_Type>(), value_type()) { }

This constructor is not implemented for legacy_variant. In my case a simple implementation like

      template <size_t Index, typename T>
      explicit variant(etl::in_place_index_t<Index>, T const& value)
        : type_id(Index)
      {
        ETL_STATIC_ASSERT(Type_Id_Lookup<T>::type_id == Index, "Missmatched type");
        ::new (static_cast<T*>(data)) T(value);
      }

did the job.