Add speed limit to Roads

[dabreegster]

I need this for acteng/atip#69 (so CC @Sparrow0hawk and @Pete-Y-CS).

I'm choosing an implementation that has less magic/guessing than https://github.com/a-b-street/abstreet/blob/6c44da83d36d0b3197525b8ca834e57a66493d24/map_model/src/objects/road.rs#L246, and also one much simpler (in terms of error reporting especially!) than https://github.com/a-b-street/osm2lanes/blob/0c166e5917748cd2dcb6eb3b798cf38136e776e6/osm2lanes/src/metric.rs.

In the future, it'd be awesome to integrate with https://github.com/westnordost/osm-legal-default-speeds.

[tordans]

JFYI, for our Project https://radverkehrsatlas.de/ we also process maxspeed (https://github.com/FixMyBerlin/atlas-geo/tree/develop/app/process/maxspeed, focus is Germany only, for now) and have a very basic fallback when no exact value is given but one of the many source/zone tags are present https://github.com/FixMyBerlin/atlas-geo/blob/develop/app/process/maxspeed/MaxspeedFromZone.lua.

[dabreegster]

Thanks for the pointer! In the short term based on what's needed in ATIP (my current work project, UK focused), we might have a similar UK-specific fallback, or maybe invest in using osm-legal-default-speeds directly or indirectly.