The circus came to the Watermill Center on Saturday. There were acrobatics, juggling, and feats of strength and balance, all of which left the audience awestruck. But because Sweden’s Cirkus Cirkor is a contemporary circus with roots in experimental dance, theater, film, music, and visual art, the group’s open rehearsal of a work in progress was a mesmerizing, haunting, and entertaining evening quite unlike the circus most of us grew up with.