Update C++ style guide (#835) - Explicitly ban `long double` - Use absl formatting libraries or `std::ostream` over printf-style functions. - Portability: use serialization libraries instead of copying the in-memory representation. - Update guidance to use `uintptr_t` (previously `intptr_t`) when working with pointers as integers. - Ban C++20 modules. - Ban coroutines (though this is expected to be temporary). - Minor wording updates.