taocpp-json 1.0.0-beta.12

• Require C++17.
• Based internal structure on std::variant.
• Based binary views on std::span (when available) or tao::span (fallback).
• Finished JAXN parts parser.
• Fixed compilation with MSVC.
• Switched CI system to GitHub Actions.
• Updated embedded taocpp/PEGTL to version 3.1.1.
• Many small improvements.
• Many small changes.
• Many small fixes.