Function hash_join 

pub fn hash_join<T, K, E, L, R>(
    left: &[T],
    right: &[T],
    include_unmatched_left: bool,
    include_unmatched_right: bool,
    left_key: L,
    right_key: R,
) -> Result<JoinPairs<T>, E>
where
    T: Clone,
    K: Eq + Hash,
    L: FnMut(&T) -> Result<K, E>,
    R: FnMut(&T) -> Result<K, E>,

Execute a generic hash join for equi-joins.

Callers provide key extraction so SQL-specific value and error types remain outside alopex-core.