[xoptional] String representation for missing values

[SylvainCorlay]

Currently, ostream operator() prints N/A for missing values, which cannot be used as an input for initializing missing values.

Maybe printing xt::missing<T> with the proper string representation of type T would be a better choice.

(Moved from https://github.com/QuantStack/xtensor/issues/381)

[SylvainCorlay]

There was a suggestion by @wolfv about a tag value castable to a missing xoptional.

Also, std::optional has std::nullopt_t.