- removed submodules in `external/`
- vendored `Catch2` and `nlohmann/json directly`
- updated conformance test generator to look outside the repo for test sources
Yeah so it turns out meson supports splitting compound boolean logic over multiple lines. Duh. Of course it does.
Also enabled ubsan for CI example builds.
Windows containers in GitHub Actions are simply bad. While it is
possible to use them, it is a horrible hack with terrible usability.
It was a fun experiment, but I don't want anybody to maintain this mess,
especially when you can use a normal Windows image (less cool, bloated,
but well supported).
