According to Merriam-Webster, a nightcrawler is just an earthworm used for fish bait. There’s nothing particularly menacing about that. But Lou Bloom, the subject of writer-director Dan Gilroy’s smart, semi-profound new thriller, is no wriggling victim. He’s a predator and a parasite, a bottom feeder who’ll find you at your lowest point and bite you in the butt as he takes your picture.