E3-669: Formatting

This MR adds clang-format to the pre-commit configuration as well as formats all source code with clang-format.

No args have been specified (in fact, it doesn't seem possible; maybe because the hook repo we use already applies agrs), and no configuration file has been added. This means that the style we apply will default back to LLVM. If we'd rather use Google's style, we would need to add a configuration file for this.