Implement iterator for edges in "Gosper Island"

[ajfriend]

Implement an iterator (or something like it) to directly provide the H3 directed edges making up the outline of the child set of a given cell.

This would provide an additional speedup for the new cellsToMultiPolygon algorithm:

1. If a user provides a pre-compacted set, we can run something like uncomopactCellsToMultiPolygon to get the outline of the cell set of all the sells uncompacted to a certain resolution. This would be faster because we can skip processing all the internal edges of the compacted cells.
2. Alternatively, if a user provides an uncompacted/flat set of cells, we could compact it as an initial preprocessing step of cellsToMultiPolygon. My guess is that in many cases, this will be faster than processing the flat set directly.

Side note: It'd be nice to have a good name for this shape. I've found "Gosper Island" (https://en.wikipedia.org/wiki/Gosper_curve), or "flowsnake (a spoonerism of snowflake)", but that might apply more to the space-filling curve than the outline.

[grim7reaper]

I've implemented an optimization like that one year ago (cf this commit)
But it's not based on clever math, so I you manage to implement something smarter, I'm interested 👀

[ajfriend]

Nice! I do have an iterator that provides the boundary edges directly, without needing any processing of the internal edges. Adding that will be one of the next steps after #1113 lands.