A sharp and illuminating answer to nearly every losing-virginity teen comedy in existence, debut feature filmmaker Molly Manning Walker’s “How to Have Sex” follows three teenaged British friends, all waiting to hear about their exam results, on holiday in Malia, Crete. It’s a spring break Cancun equivalent: a nonstop barrage of all-you-can-drink, clubbing, shots, puking, more shots, more clubbing, 4 a.m. chips (fries) and hookup options by the score. In the breathless early scenes, we scramble along with Em (Enva Lewis), Skye (Lara Peake) and Tara, also known as Taz, played by the remarkable n…