Integrate Gosper iterator into cellsToMultiPolygon

[ajfriend]

This PR provides the code I was using to test out incorporating the Gosper Iterator from #1138 into the cells_to_polygon algorithm, which should provide a significant speedup for compacted cell sets.

I'm leaving this here for future reference for someone to take over from here.

What we still need to think through is how this will be reflected in the API. We only get speed benefits on compacted sets. Do we take flat sets and compact them under the hood automatically? Do we have different functions that expect flat vs compacted inputs? What validation do we do, and how do we handle duplicate cells or descendants of cells being present?

[coveralls]

coverage: 97.713% (-1.4%) from 99.162% — ajfriend:gosper_iter_and_cells2poly into uber:master