Kani needlessly recompiles everything every time

[Kixunil]

When trying to get kani to work one often has to do small changes to a file. Kani recompiles all crates all over again even when only one file changed. Including proc macros, so if there's some technical reason why the code can't be reused for conversion to CBMC at least proc macros should be reusable.

[zhassan-aws]

Hi @Kixunil. This is indeed very annoying. We've had to disable incremental compilation because of a few bugs that we're working on fixing. This is tracked in #1736.