Handle implicit oneways

At the moment our destination and exit number re-writer only re-writes if the ramp is a oneway by means of having a `oneway=*` tag.

There are two implied oneways:
- [`junction=roundabout`](http://wiki.openstreetmap.org/wiki/Tag:junction%3Droundabout)
- [`highway=motorway`](http://wiki.openstreetmap.org/wiki/Tag:highway%3Dmotorway)

we should check in addition.

These are edge cases but for correctness sake we should handle them anyway.

No file or test is named in the issue. Start by locating the destination and exit number re-writer and inspect how it detects the oneway tag; done means ramps with junction=roundabout or highway=motorway are also rewritten as implicit oneways, with relevant tests updated or added if the project has them.

Written by the indexing model from the issue text.