The competition among low-calorie ice creams is heating up. Ben & Jerry’s, known for its ice cream filled with chunks of fudge, brownies or cookie dough, is the latest big brand to launch a slimmed-down version of its frozen treats. It’s an effort to better compete with Halo Top, a low-calorie, low-sugar ice cream that is growing fast and sending shivers through older brands. Halo Top’s entire pints have less than 400 calories, and it emphasizes the full calorie count on the front of its package — a move others are mimicking.