use enter + exit for roundabout instructions (#4358)

* Expose roundabout/rotary exit instructions as a new instruction type.

include/engine/api/route_api.hpp

@@ -167,12 +167,12 @@ class RouteAPI : public BaseAPI
                  * the overall response consistent.
                  *
                  * ⚠ CAUTION: order of post-processing steps is important
-                 *    - postProcess must be called before collapseTurnInstructions that expects
-                 *      post-processed roundabouts without Exit instructions
+                 *    - handleRoundabouts must be called before collapseTurnInstructions that
+                 *      expects post-processed roundabouts
                  */
 
                 guidance::trimShortSegments(steps, leg_geometry);
-                leg.steps = guidance::postProcess(std::move(steps));
+                leg.steps = guidance::handleRoundabouts(std::move(steps));
                 leg.steps = guidance::collapseTurnInstructions(std::move(leg.steps));
                 leg.steps = guidance::anticipateLaneChange(std::move(leg.steps));
                 leg.steps = guidance::buildIntersections(std::move(leg.steps));

include/engine/guidance/post_processing.hpp

@@ -17,7 +17,7 @@ namespace guidance
 
 // passed as none-reference to modify in-place and move out again
 OSRM_ATTR_WARN_UNUSED
-std::vector<RouteStep> postProcess(std::vector<RouteStep> steps);
+std::vector<RouteStep> handleRoundabouts(std::vector<RouteStep> steps);
 
 // trim initial/final segment of very short length.
 // This function uses in/out parameter passing to modify both steps and geometry in place.