Continue the Rust implementation

[dabreegster]

• Improve performance (parallelize, reconsider the conversion to geo, check parsing and writing)
• Come up with a simple way of sanely viewing the output to manually validate. geojson.io and similar aren't good for spotting adjacent line-strings; probably make a custom Leaflet style
• Optionally split when some property (an OSM ID) changes
• Ignore direction (optionally?)
• Split line segments when needed
• Validate to the R implementation's output, and compare performance

[dabreegster]

For perf, keep an eye on georust/geojson#223

[dabreegster]

Converted into smaller issues