Adopt clang-format for all files?

As part of adding lint style checks in #2369 (closed) we considered using clang-format but balked at reformatting every single file and thus adding a history hurdle. But as more contributions come in with style issues and as I see other projects successfully use clang-format to simplify things, I lean more and more toward biting the bullet and reformatting everything and then adding a pre-commit check.