First take at some tests for the new verify_single function, ala #113

This completes an implementation of mocks for the core types for the lite module and shows how to use them for testing. Still unmocked are the Store and Requester.

There's probably much to be improved here, including the test names and how we organize them, but this seems like a decent start.

This also surfaced some issues with error types:

• when used to verify a commit (rather than just checking if we can skip), voting_power_in can't detect signatures from validators not in the current validator set
• we should have two distinct errors for InsufficientVotingPower - one when it's insufficient to skip, and one when it's insufficient to verify! Otherwise we might start bisecting when really the commit is just invalid ...

For now we're just duplicating some of the mocks that are already in the verifier tests module. Probably verifier.rs and public.rs will be consolidated down to verifier in #114 and this will be deduplicated then.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGES.md