Index roads instead of segments

[benjamintd]

Fixes #21

Instead of indexing and returning individual segments, we index the full road and return the index of the matching segment.

We avoid creating many segment objects in memory, and reduce the size of the index, at the cost of having larger bounding boxes in the index - thus potentially more candidates to check when matching a point.

The benchmark runs on average ~20% slower, although I suspect the effect would be less obvious on real datasets with larger indices.

Note that this creates a breaking change in the API, since we're not returning segments anymore.

[tcql]

Looks good! The performance hit is a bummer, but I see the improvement from a memory management perspective.

[benjamintd]

Thanks @tcql for reviewing!

Using this branch in a production setting didn't show a noticeable performance drop, however memory usage was cut in half. I think it's good to merge.