Update C++ style guide (#971) - Ban forward declaration of entities not owned by the project. - Recommend `absl::down_cast` for downcasting instead of `static_cast`. - Expanded and reworked guidance on `auto` for variables and function parameters: in particular, encourage structured bindings for map-like containers. - Remove Boost section and add a section for third-party libraries with general guidance. - Other minor updates and improvements to wording, et cetera.