Add an explicit conversion operator from rust::Str to std::string_view.

[anforowicz]

PTAL?

I am aware of #80 (adding a way to construct rust::Str from std::string_view, using a custom C++17-detection mechanism) and #410 (adding a way to construct rust::String from std::string_view, also using a custom C++17 detection mechanism). But maybe we can still proceed with this PR given that:

• This PR is relatively simple - just adds one member (a conversion operator) to rust::Str. I think that adding such an explicit operator has close to zero risk of breaking existing users (unlike the other PRs where new constructor overloads may risk ambiguity or other resolution problems)
• Since the other PRs were raised, we were able to proceed with other C++-version-dependent APIs:
  • Tweak rust::Slice to become a C++20 contiguous_range. #1432 added a (C++-20-specific) way to convert rust::Slice into Chromium's base::span (which needed rust::Slice to expose a different iterator_category in C++20 than in earlier versions).
  • Cover C++20 in CI (fixing char8_t-related problem there). #1437 added CI coverage for different C++ versions.

FWIW I've also noticed 5ae3a93 where #if __cplusplus >= 202002L was replaced with #ifdef __cpp_char8_t. IIUC this is using https://en.cppreference.com/w/cpp/utility/feature_test which are available in C++20 and later. So I think we can't do that in this PR.

[dtolnay]

Thanks!