Add `c-testsuite` CN examples

[septract]

• Adds a new directory src/example-archive which we can use for CN examples
• Adds a first example corpus, c-testsuite, 220 small C programs
• Categorizes the examples into working / broken
• Documents the schema for how examples should be categorized into subdirectories
• Adds a helper script for checking that examples are correctly categorized.

[septract]

@cp526 Would you be able to review and merge this?