NEW YORK — It’s going to be one hot spring. At least, that’s what designers seem to think. Several collections shown here last week featured swimsuits — and they weren’t for the faint of heart. “It’s all part of the super sexy look,” said Cynthia Rowley, who showed suits with strategically placed cut-outs. “It’s not just a swimwear thing anymore. You mix it up with other clothes all year round.” And that’s exactly what designers like Victor Alfaro, Michael Kors and Ralph Lauren did, showing such looks as a bikini bottom paired with a long shirt, or a bikini top with hip-hugger pants.