[ refactor] derive under elab so that additional queries and error reporting can be done

[MarcelineVQ]

While working on some tooling using elab-util I've found I need more power available at the call site for queries about what's in scope and for the ability to report errors along the way, updating derive to return a value under Elab was the most direct solution I could find to the problem.

[MarcelineVQ]

re https://github.com/MarcelineVQ/blessed-derive

[MarcelineVQ]

Decided to keep these changes relegated to my own lib for now, as the lib has been rapidly changing