Add `StopAreasToWayCostsConverter`

A new class that converts a list of `StopArea`s to `WayCost`s. This will
be called from `Chouette::Route` to get `WayCost`s from its stops, which
will then be JSON serialised and stored in `Route#costs`.

Update `WayCost`:
* Remove comment about calculating the ID automatically. It actually
  needs to be the same as the `JourneyPattern#cost` ID (`key`), which is
  a string with the IDs of the departure and arrival stops.
* Make `#==` check that `id`s are the same, which necessitates making
  `id` a reader.

Refs #6095

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