To make filenames in a future commit less ambiguous.

This commit switches the data validation package from `cerberus` to
`schema`. This is because schema is able to emit the schema in a
standardized format that can later be rendered into HTML. This will be
implemented in a future commit.

This commit adds the schema of benchcomp.yaml to the Kani book to
document how benchcomp configuration files are written. The code will be
used in future commits for documenting the parser and visualizer
formats.