Hashtable

For example, using the 797M shp/us/zipcodes.shp as input:

```bash
time make topo/us-zipcodes.json
mkdir -p topo/ && node --max_old_space_size=8192 node_modules/.bin/topojson -s 3e-7 -- shp/us/zipcodes.shp | ./topomerge zipcodes > topo/us-zipcodes.json
quantization: bounds -176.684744 -14.373764999999999 145.83050500000004 71.34132400000001 (spherical)
quantization: maximum error 1.804km (0.0162°)
simplification: retained 442110 / 1511624 points (29%)
prune: retained 187688 / 198243 arcs (95%)

real	1m29.911s
user	1m27.554s
sys	0m3.240s
```