Add std::wistream support.

[deiuch]

• Describe the feature in as much detail as possible.
  For now, parser (specifically detail::input_adapter) can work only with std::istream and std::wstring (and some other string types), but I also need a support of wide character input stream. I hope that would be easy since there is already support for wide character strings in the library.

[nlohmann]

That should be possible. PRs welcome!

[stale]

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