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.