sparse arrays for the chart

[anoopsarkar]

The chart is currently allocated and de-allocated for each sentence. Initialization also takes time proportional to the number of cells in the chart. We should use a sparse array trick where a single chart is allocated with a max size and for each sentence we use a sub-part of the entire chart. Also we should implement the O(1) clear operation for sparse arrays.

http://research.swtch.com/sparse