Combination Sum

Write `combination_sum(candidates, target)` returning every unique combination of distinct candidates summing to the target. Each candidate may be reused any number of times. Order within and between combinations does not matter. combination_sum([2,3,6,7], 7) -> [[2,2,3], [7]] Reuse means recursing with the *same* index, not the next one. Uniqueness means never looking backwards: pass a start index and only consider candidates from there on, or you will emit [2,2,3] and [2,3,2] and [3,2,2].