Clean up public API to replace const char*, size_t to std::string_view, union to std::variant etc

We need to clean up the public API before the release of 0.17.0. Here is what needs to be done:

• replace const char*, size_t parameter pairs with std::string_view,
• replace pstring as parameters with std::string_view,
• replace union with std::variant, and
• move the pstring header to private place to avoid advertising it to public.

For now, pstring may still need to be exported since replacing it with std::string_view wholesale will require a huge amount of effort. But pstring should eventually become invisible and eventually be removed in favor of std::string_view.