Refactor Roundabout Handler

[MoKob]

The roundabout handler should see some refactoring.

We should tackle the following tasks:

• make handler work for left sided driving and ride sided driving (it might suffice to simply use turn angles in mathematical negative direction over positive direction) (possible impact here
• remove own coordinate calculation in favour of Improve Turn Angles #2779
• handle traffic lights / sliproads on exits (https://github.com/Project-OSRM/osrm-backend/blob/master/src/extractor/guidance/roundabout_handler.cpp#L169)
• maybe throw in Detect Segregated Ways at Roundabout Turns #2943
• revisit distinction criteria (here)
• refactor roundabout type detection here
• refactor roundabout handling here
• finally split up into enter/exit roundabout instruction
  • cleaner handling in post-processing
  • far less error prone
  • actual location of exit will be known to application
  • distances / names can be fixed: rotary_name can be removed

[TheMarex]

Description of the items here is either too unspecific to warrant an issue, outdated, or covered by other issues. Closing as non-actionable.