Add legacy examples from the `VERSE-Toolchain` repo

Add simple-examples, which contains examples from VERSE-Toolchain/cn-intro/simple-examples.c broken down into individual files and categorized.
Add dafny-example, which contains examples from VERSE-Toolchain/cn-intro/dafny-examples.c broken down into individual files and categorized.
Add new crashing cases to simple-examples/broken/error-crash/. These were constructed using the creduce tool.
Update the checking script check.sh to handle cases where target directories are empty / nonexistent.

rems-project / cn-tutorial