Function nested_loop_join 

pub fn nested_loop_join<T, E, P>(
    left: &[T],
    right: &[T],
    include_unmatched_left: bool,
    include_unmatched_right: bool,
    matches: P,
) -> Result<JoinPairs<T>, E>
where
    T: Clone,
    P: FnMut(&T, &T) -> Result<bool, E>,

Execute a generic nested-loop join.

include_unmatched_left and include_unmatched_right model LEFT/RIGHT/FULL outer-row emission while leaving concrete join-type dispatch to callers.