Add Gosper island boundary edge iterator

[ajfriend]

Closes #1114

Adds IterEdgesGosper, an iterator that yields the directed edges forming the boundary (Gosper island outline) of a cell's child set at a given resolution.

• Each edge's origin cell is a child of the parent cell; its destination cell is not
• Edges form a closed counter-clockwise loop
• For a parent at resolution r expanded to child resolution R: hexagons produce 6 * 3^(R-r) edges, pentagons produce 5 * 3^(R-r)
• For now, this is an internal-only iterator, not included in the public API

Usage

IterEdgesGosper iter = iterInitGosper(cell, childRes);
while (iter.e) {
    // iter.e is the current directed edge
    // iter.remaining is the number of edges left
    iterStepGosper(&iter);
}

Next step: Speed up cellsToMultiPolygon

This is a building block for a faster cellsToMultiPolygon. When given a pre-compacted input cell set, the iterator allows the cellsToMultiPolygon algorithm to walk the Gosper island boundary edges directly, skipping all internal edges. On a Colorado polygon at resolution 8, this yields a 35x speedup. The higher the resolution, the better the speedup. This speedup is separate from the 3-4x improvement already achieved with the new cellsToMultiPolygon algorithm.

[coveralls]

coverage: 99.081% (-0.01%) from 99.095%
when pulling d96092e on ajfriend:gosper_iter
into 1d5346b on uber:master.

[ajfriend]

Talk explaining how this fits in: https://ajfriend.com/h3c2p-talk/

[isaacbrodsky]

You could also write this with type Direction and use I_AXIS_DIGIT, etc.

[ajfriend]

Yeah, that's better. Updated.

I left some comments on the numeric sequence, because I think that makes it easier to see the shared pattern.