Add block level messages

[trktby]

This PR aims to provide more granular progress reporting for the Prusti-Assistant VS Code extension. It is part of a practical work supervised by @Aurel300. It depends on a corresponding Silver PR.

The new Silver messages are emitted during execution. New information for tracking progress is added to the Executor and State, and kept up to date if a new --generateBlockMessages flag is set. This information includes:

• an ID for execution paths (a new ID is assigned at a branching point)
• a hashset to keep track of the completion of such paths
• a currentBlock field in the State, comprised of a label and path id.