tile indexing

[morganherlocker]

An OSM object should be included in any tile extract if any of its nodes are contained in that tile. We can vendor the tilebelt implementation of this tile labeling operation.

This approach will be extremely fast (~10M nodes per second in JavaScript and possibly much more in C++), and provide "good enough" tile indexing. Where it will fail is when an edge crosses over a particular tile, without any nodes being in that tile (like at a corner). I think we can overlook this for the types of data analysis we are concerned with. If we find that we realllly don't want to make the tradeoff, we can implement tile-cover in C++, although that would be a bigger lift.

An open question is what we should do with giant geometries like coastlines that span large numbers of tiles. I would be in favor of logic that says something like "if an object touches >100 unique tiles, bail and do not include it". Coastlines and other giant objects do not fit the TileReduce scaling model well, and are not needed for any types of analysis we need so far. If a particular processor does need these, it can handle these geometries on its own.

cc @rclark

[morganherlocker]

Done #17