C++Now 2019 design

• The design as described on C++Now 2019 talk (https://youtu.be/wKchCktZPHU)
• Added C++20 features supported by gcc-9.1 (std::remove_cvref_t, down with typename, std::type_identity)
• Compile-time performance optimizations (type_list, common_ratio, ratio, conditional_t)