Implement highway conflation

[rsavoye]

This implements highway conflation, currently tested with several external highway datasets. The algorithm takes two GeoJson FeatureCollection or OSM XML files, and returns a GeoJson FeatureCollection. This enables it to be used in a backend. It is heavily multi-threaded, but brute force, so not fast for large datasets, but quick for small datasets. This matches both highway geometry and tagging to identify features.

The standalone version writes both a GeoJson and OSM XML file for editing with JOSM or QGIS. For the OSM XML version, tags are automatically merged with existing features for faster validation. While this is still a bit of a work in progress, the results I've gone through are good, and I've uploaded some of it to OSM already.