Go further on the road to using C++20

What's Changed

• Use hidden friends, defaulted comparison and three-way comparison by @YexuanXiao in #34
• Simplify get_activation_factory by @YexuanXiao in #33
• Use concepts instead of enable_if and void_t by @YexuanXiao in #35

Full Changelog: v2.1.260306.1...v2.1.260314.1