feature: elsa for std string types

[impugachev]

This will allow using frozen containers with std::string_view(C++17) and std::string(C++20). I'm not sure if tests are needed for this ... I plan to add a heterogeneous search in the next PR, there it will already be possible to write tests that will use the hash for std::string_view.

[impugachev]

@serge-sans-paille could you please take a look?

[serge-sans-paille]

Thanks for the patch - I'm fine with the idea. I'll add C++20 to the validation then you can add a test to make sure this indeed works fine.

[serge-sans-paille]

Validation now supports c++20. You can test with std::string container on C++20. Please rebase on master branch ;-)

[impugachev]

I'm not quite sure what test to write here, so as not to fall into testing the obvious things ... Therefore, I added just one simple test to check compilation

[serge-sans-paille]

I was thinking about testing, say, a frozen::set of std::string. That's what we want in the end, right?

[impugachev]

Ok, I will then add a typed test for searching in frozen containers

[serge-sans-paille]

Looks good, can you rebase on master branch and squash all your commits in one?

[impugachev]

Done @serge-sans-paille, can you please merge?

[serge-sans-paille]

Thanks for your patience o/