Improvements to chart

Improvements to chart

  * queries (= Chart.__getitem__) perform fewer set intersections (removed a
    very slow call to set.copy -- it's faster to intersect two sets and get a
    copy because the intersection is guaranteed to be smaller!)

    I'm also using a most constrained variable first heuristic for picking the
    first index to intersect.

  * fewer hash look ups with we call ``insert``.