History is so packed with conflicts that turned into humanitarian disasters that some tend to be overlooked, or at least underemphasized. I’m not sure how much today’s high school students learn about the civil war that raged in Sri Lanka between 1983 and 2009 — or its long and complicated lead-up, for that matter — but as is made clear in S. Shakthidharan’s sprawling, haunting new play, “Counting and Cracking,” it can’t be enough.