PrivCount proof of concept: work out how to do integration testing without chutney

Tor and chutney can be unreliable, so we might need to do integration tests on PrivCount without chutney.

Here's one possible design:

• split PrivCount into generic rust modules, tor-specific rust modules, and tor interface modules
• create a command-line interface to the generic rust modules and tor-specific rust modules, which takes events, writes results, and aggregates results from files
• use these file-based interfaces to integration test the non-tor, non-network PrivCount code