Split "codegen" flags to reduce recompilation

[celinval]

Requested feature: Only recompile the artifacts that are affected by changes to Kani options.
Use case: After enabling the build cache, any change to the Kani arguments that influence how we invoke kani-compiler will trigger a full recompilation. This is unnecessary in some cases (e.g.: reachability checks), which are only applied to the target crate being compiled.