A good epic poem is hard to come by these days, and it’s not just due to our kitten-like attention spans that there aren’t more of them out there. The real challenge awaiting any would-be contemporary American epic poet is the impossible multiplicity of our history - how to tell our story without leaving everyone out. Twenty years in the making, Kevin Young’s “Ardency’’, rises fearlessly to the challenge of historical poetry, in both the breadth of its scope and the intimacy of its materials.