renamed ProofContext.theory(_result) to ProofContext.background_theory(_result) to emphasize that this belongs to the infrastructure and is rarely appropriate in user-space tools;

standardized Context.copy_thy to Theory.copy alias, with slightly more direct way of using it;

merged

merged

consider "locality" when assigning weights to facts

add a bonus for chained facts, since they are likely to be relevant; (especially in a Mirabelle run!) -- chained facts used to be included forcibly, then were treated as any other fact; the current approach seems more flexible