feat: add map combinator for input projection

[leno23]

Summary

Adds MapPredicate and PredicateMapExt::map to adapt a predicate on one type (e.g. TaskId) for use on a larger type (e.g. Task) by mapping the input before evaluation.

Addresses the use case from #142:

let has_id = predicate::in_hash(ids).map(|task: &Task| task.id);
predicate::always().boxed().and(has_id.not()).boxed();

Test plan

• cargo test (unit + doctests)
• map_in_hash and map_combines_with_boxed regression tests

Fixes #142

Made with Cursor

[epage]

This was in a "handling of feedback" mode.