Switch to DuckDB for prototype

[hamilton]

DuckDB is likely much better-suited than sqlite3 for the task at hand. Let's see if it can provide the same needs:

• a way to run the query (this is obviously easy)
• a way to get info about the query, such as an execution plan that alludes to the source tables

[hamilton]

After a bit of exploration, it looks like DuckDB does a great job of standing in as a backing db. I have a PR together that refactors to utilize it.

To make this feel a bit more real, I think we'd have to:

• clean up the API I've built so we can standardize the application on it (making the engine a bit more pluggable)
• wrap all the relevant duckdb code into its own module