[debug] Add manticore::debug

[mcy]

The debug module is intended to hold debugging helpers, to make debugging Manticore tests easier. This includes:

• Redactable logging.
• Error creation capture (e.g., capturing where Err variants are created).

The generated logs can be captured by tests, and eventually get returned through Cerberus's "debug log" functionality.

Right now, manticore::Result is unused. If we decide to go with this design, we'll need to manually update a ton of callsites to use it, and i'm not certain this can be done gradually. I think this will ultimately be worth it, since I won't have to break out GDB to do basic debugging of unit tests. Other than this immediate benefit, it means we can extract fine-grained errors out of Manticore for other purposes.