Add `cellsToMultiPolygon` function

[ajfriend]

This is a tracking issue for sub-issues/PRs related to adding a "cells to polygon" function like:

H3Error cellsToMultiPolygon(const H3Index *cells, const int64_t numCells, GeoMultiPolygon *out);

This function should fix #1048, #1049, and #1051, while being faster for large cell sets, and allowing us to handle "global" cell sets that produce polygons that cover a majority (or all) of the globe and/or cross the poles or antimeridian.

Compare this function to cellsToLinkedMultiPolygon, which returns a LinkedGeoPolygon. Once working, my plan would be to reimplement cellsToLinkedMultiPolygon as a light wrapper around cellsToMultiPolygon. Though, I would expect most downstream users would prefer cellsToMultiPolygon since GeoMultiPolygon is easier to work with; for example, we'll be able to remove a bunch of code from h3-py when we no longer have to walk through the LinkedGeoPolygon in the binding.

Sub-Issues and PRs

To avoid one giant PR, I've been trying to break this up:

• Add reverseDirectedEdge #1098
• Add geoLoopAreaRads2 function #1101
• (Multi)polygon helper functions #1104
• cellsToMultiPolygon core algorithm #1113
• TODO: review *.c/*.h file organization after all functionality added, as mentioned in Add geoLoopAreaRads2 function #1101 (comment)
• Consider raising error when edge arcs close to 180 degrees are encountered #1108
• Consider alternative construction of globe multi polygon #1110
• consider a mode flag, for things like skipping input validation
• API consideration: should uncompactCellsToMultiPolygon just completely subsume cellsToMultiPolygon, so we only need the former?
• Implement iterator for edges in "Gosper Island" #1114
• Consider using sorting instead of hashing to identify symmetric edge pairs #1117
• Replace cellsToLinkedMultiPolygon implementation with cellsToMultiPolygon #1127

[ajfriend]

I'd also like to add a function like

H3Error uncompactCellsToMultiPolygon(const H3Index *cells, const int64_t numCells, int res, GeoMultiPolygon *out);

which can take a (fully or partially) compacted set of cells, uncompact them to the given target resolution, and return the resulting multi-polygon.

Discussed last Thursday. Folks seemed to prefer the name uncompactCellsToMultiPolygon, rather than something like cellsToMultiPolygonAtRes.