Previously, this script ran over every C file in our source code,
which took up to a minute on my desktop.
Instead, the script now has several modes that it can run in, to
minimize the time spent and improve useful output. It should now be
suitable for everyday use and git hooks. I hope.
I've also renamed this script, so that we can keep using it in the
future if we were to move to some tool other than clang-format.
