Intelligently default --jobs and --split

[sorear]

--jobs should be the logical CPU count unless overridden (hopefully there's a crate for this).

--split should be enabled automatically if --jobs > 1 or --repeat. We may need --no-split --repeat, although that's a pathological case only really useful for profiling.

[digama0]

There's a crate for this: https://crates.io/crates/num_cpus

[sorear]

Thought so. Thanks for finding that!