Extract some code from `#batch` to allow us to test the part that takes
distance and time values from the response JSON and put fill them into
`WayCost`s.

In order for the test to work, had to add an `#==` method to `WayCost`
and make `distance` and `time` accessors.

The JSON file fixture is a copy of a response from the TomTom `/batch`
API. The file is pretty big. I'm not sure if maybe I should condense it
for the sake of test performance.

Refs #6095

Rename to `#convert_way_costs` since this method now lives inside the
`TomTom::Batch` class, so that part of the name became redundant.

Refs #6095

This change enables us to call `TomTom.batch(...)` instead of
`TomTom.new.batch(...)` a couple commits ago. This is nice because the
initialisation was kind of unnecessary for users of the class.

Refs #6095

Change the `TomTom::Batch` to `module ... class ...` syntax instead. I
like this better in this case.

Refs #6095

Separate the functionality a little better by moving the `/batch`
endpoint code into a new class. The goal here is also to lay the
foundation for being able to call `TomTom.batch()` instead of
`TomTom.new.batch()`.

Refs #6095