export-validator does not support Windows

[hannobraun]

export-validator is an application in this repository that uses the Fornjot app to export all models in the models/ directory to 3MF files, and to validate the correctness of those 3MF files (using lib3mf). This is extremely valuable, as it catches a lot of bugs as part of our extended test suite. Specifically, export-validator is run on every CI build, and can be run locally via just build.

lib3mf is a C++ library, which makes the whole setup a bit problematic. I figured it would be easiest to just use the pre-compiled binaries from their SDK. This works for Linux and macOS, but I failed to get this to work for Windows, meaning export-validator doesn't run on Windows at all. This is a problem in two ways:

• Windows developers can't use just build or cargo run -p export-validator locally, which limits their ability to test their changes without submitting a pull request and running a CI build.
• The CI build has reduced test coverage on Windows, which might allow platform-specific bugs to slip through.

For these reasons, it would be great if export-validator supported Windows too. In principle, this would involve the following steps:

1. Extract the pre-compiled Windows libraries from the lib3mf SDK.
2. Add those files to the libs/ directory under the correct names, so they can be found when linking.
3. Adapt export-validator's build.rs so everything works.
4. Enable export-validator in the Windows CI build, by removing the conditional that prevents it from running.

I tried to do this, but I just couldn't get it to work. I managed to link to the DLL, but I couldn't make it so that the DLL would then be found at load-time (i.e. when export-validator runs). Here is my aborted attempt: #1230

I would really appreciate some help with this. I don't have Windows, don't know a whole lot about it, and testing this only via CI builds is a pain in any case. Labeling as https://github.com/hannobraun/Fornjot/labels/help%20wanted.