Use C++20 features (was: Evaluate std::string_view)

[jhasse]

Probably has to wait until C++20: https://stackoverflow.com/a/53530846/647898

[jhasse]

load could also return std::jthread.

C++20 (std::jthread in particular) compiler support:

• [x] GCC 10: https://en.cppreference.com/w/cpp/compiler_support
• [ ] Apple Clang
• [x] MSVC
• [ ] Android's Clang
• [ ] Nintendo Switch
• [ ] iOS

[jhasse]

Use https://en.cppreference.com/w/cpp/numeric/constants instead of M_PI