Add script to compare core bazel and CMake headers

[chiphogg]

This gives us the most important portion of #255. In fact, it's even
better than what we originally suggested, which was a blocking checkbox
in code review. This solution uses bazel query to get the
authoritative set of headers included in our main targets, //au and
//au:io. It compares them to the CMake target au, which encompasses
both of these targets.

The script returns an exit code of 0 if the headers are an exact
match, and 1 otherwise. If any files differ, it prints out the list
of every file that is found in only bazel or only CMake.

A good way to test this is to run:

check-cmake-headers && echo $?

This should print 0 on the last line. Then, if we edit
au/code/au/CMakeLists.txt, and change the name of one of the files
(say, prepend asdf), we should see the discrepancy printed out, and
the printed exit code should change to 1.

A future PR will add a GitHub action to call this script and gate PRs on
its result.

[chiphogg]

For proof that the script will actually work in CI, see this passed build for 1805ffa (which temporarily made it gating), and this failed build for 82e6727 (which intentionally created an error condition).

[hoffbrinkle]

I tend to lean towards long arguments when using scripts to make them more self documenting. Not being a regular cmake user I have no idea what -B and -S do. Well, I have have a decent guess as to -S.

[chiphogg]

Interestingly, cmake --help didn't seem to suggest a long-form alternative!

Usage

  cmake [options] <path-to-source>
  cmake [options] <path-to-existing-build>
  cmake [options] -S <path-to-source> -B <path-to-build>

Specify a source directory to (re-)generate a build system for it in the
current working directory.  Specify an existing build directory to
re-generate its build system.

Options
  -S <path-to-source>          = Explicitly specify a source directory.
  -B <path-to-build>           = Explicitly specify a build directory.
  -C <initial-cache>           = Pre-load a script to populate the cache.
  -D <var>[:<type>]=<value>    = Create or update a cmake cache entry.
  -U <globbing_expr>           = Remove matching entries from CMake cache.
...

Anyway, I don't know them from memory either, but this command was basically pasted from our installation instructions. I've added a link here in a comment; thanks for pointing it out!