Add a (default) timeout feature to kani/cargo-kani

[tedinski]

If the solver is non-terminating, we should have a better experience than simply running forever. Now that we have a Rust implementation of kani and cargo-kani we should introduce this feature (with a default timeout that can be extended).

• Introduce a timeout. Decide on a default. (Random initial proposal: 2 minutes. Short enough to discover it exists quickly if you're trying to do long proofs, long enough that most proof should be fine?)
• Add an attribute (like unwind) for proof harnesses to modify this timeout.

[tedinski]

IMO, I overestimated the initial default timeout. I think we should get customers to think about timeouts for anything that's going to realistically require them. We should probably default to at little as 5-15 seconds spent in CBMC.

This information, besides being a timeout, would also be helpful for CI proof runners (e.g. finding and prioritizing "critical paths" to maximize parallelism.)

[zhassan-aws]

A timeout option was added in #3649. Closing.