Add flag to generate GOTO program

[feliperodri]

Kani now has an option called --gen-c, which generates a C file equivalent to the Rust program. We should add a --gen-goto option as well to get the equivalent GOTO program. CBMC has an option called --show-goto-functions with a similar functionality. It could be quite helpful for debugging purposes.

[kiyoakii]

I think this would be very helpful indeed.

[zhassan-aws]

For the record, this is currently possible through running Kani with --keep-temps, and running goto-instrument --show-goto-functions on the goto binary.