Concise list subcommand option that groups by module

[carolynzech]

Proposed change:
Add a --module-totals flag to the list subcommand that just prints the total contracts and harnesses per module.

Motivation:
As our standard library verification effort grows, the kani list tables get huge, and it's hard to get a birds-eye view of the progress we've made. It would be good to have the option for more concise output.