Remove `--function` option

[celinval]

Proposed change: We should include information about the public functions as part of the KaniMetadata when the reachability mode is pub_fns. We should stop merging all the metadata and goto binaries together.

Motivation: We have a special handling for --function today inside kani-driver because we cannot tell which artifacts are relevant to some function. So we just merge everything. If we treat functions as potential harnesses, we can get rid of the hack described above. We should also restrict the metadata to include functions that don't take any parameter.

[tedinski]

We could also just search the symtab files directly, it might be simpler that way.

[celinval]

Do we mean parse the symtab file or invoke some goto utility to print the functions available?

[tedinski]

The former for now, and hopefully if we're able to directly generate goto binaries that will also come later with the ability to open and parse those

[celinval]

I am leaning towards adding information to the Metadata because is more reliable and it will work well even with mangled names. Since this will only be done for the target crate, the cost shouldn't be too high.