Update C++ style guide (#914) - Expand guidance around usage of functions (and variables) with inline definitions. - Change guidance around internal namespaces to reduce the likelihood of collisions. - Expand naming guidance, with specific additional rules around: - nested namespaces not reusing well-known top-level names - template parameters - name aliases - Clarify that constant-style naming also applies to static class constants. - Other miscellaneous formatting and documentation improvements, e.g. additional Abseil TotW links.