Store consuming jobs

A stored job may need to hand its captured resources to its caller. Use the existing def once form and extract the job before calling it.

def make_job(n: i64) -> Result[OnceClosure[[], list[i64]], AllocError]:
    values = [n]?
    job = def once [values]() -> list[i64]:
        values
    Ok(job)

def main() -> Result[(), AllocError | IoError]:
    mut jobs = [make_job(10)?, make_job(20)?]?
    match jobs.pop(0):
        case Some(job):
            print(job())?
        case Nothing:
            pass
    for job in jobs:
        print(job())?
    Ok(())

[10]
[20]

Calling jobs[0]() would try to consume a borrowed entry and is rejected. Extract an owned callback with pop, consuming iteration, consuming enum matching, or tuple unpacking.

Dropping uncalled jobs cleans up their captures. Borrowed captures and generator-containing environments cannot be stored in these heap owners.