Adding support Support fmt/7.1.x, string-view-lite/1.6.0, and variant-lite/2.0.0

[prince-chrismc]

The title includes a list of RESTinio dependencies which have seen update(s) in recent weeks/months.
Not include are the PRCE and PREC2 which have also seen updates, the ones above have more integration challenges however (as the they are not optional to restinio).

• I quickly tested fmt/7.1.2 against restinio/0.6.12 and there was a complication failure with restinio::optional (I did not save the logs unfortunately)
• I have successfully used string-view-lite/1.6.0 in a project along with RESTinio without any issue.
• I have never tried the latest major release of variant, though the release notes do not suggest and API break I assume there is one.

As always any time you spent looking at this would be greatly appreciated ❤️

[eao197]

Do you use RESTinio in C++17?

[eao197]

I've updated RESTinio to fmtlib-7.1.2 in 0.6-dev-0.6.13-update-deps branch and checked it with a couple of different versions of different compilers with C++14 and C++17, and doesn't found any compilation errors.

[prince-chrismc]

I had tested in in GCC 9 which might be 17 by default, it might have been bad example code but I am glad it worked

I am trying to compile this example (slightly reduced) https://github.com/Stiffstream/restinio/blob/a6a07d147a04192cd0f590492ce6dd321597d54c/README.md#enhanced-example-with-express-router

I pushed a commit that was failing.