Make it clearer why we have to use a custom serialiser here.

Refs #6222

Rails serialises `BigDecimal`s as JSON strings to prevent loss of
precision. The `latitude` and `longitude` columns in `StopArea` are
stored as `BigDecimal`s. The trouble is that TomTom's API requires the
latitude & longitude values to be JSON floats, not strings.

Make a new JSON serialiser that converts the `BigDecimal` coordinates to
float to allow the values to be correctly interpreted by the API.

Refs #6222

Already handled this a few commits ago.

Refs #6222

Using a set ended up not working out. I needed to be able to index into
the list in `#extract_costs_to_way_costs!`, and sets aren't indexable.
This is because they're supposed to be unordered, though modern Ruby
implements `Set` with `Hash` under the hood, which is ordered in Ruby.

I like the idea of having a data structure that automatically eliminates
duplicates, but it wasn't meant to be, because for the extraction to
`WayCost`s, I need an ordered list. Rather than create a new
`OrderedSet` type, I just went the simple route and used an Array,
eliminating the duplicates manually because I know when duplicates are
supposed to occur due to the nature of the data set.

Remove the `#eql?` and `#hash` methods from `TomTom::Matrix::Point`.
Because we're not longer using `Set`, these methods don't need to be
implemented.

Refs #6222

Change this code to get stop IDs based on the change in
be3e1effcdea87909a181c7e9b12cf6867b1839d. It should use these IDs to
construct new `WayCost`s with the combination of stop IDs from departure
and arrival stops.

This doesn't work currently because the method has code that tries to
index the `points` collection, but it's a `Set`, and sets don't support
indexing and aren't ordered, so this code errors. Need to change the
`Set` to something else that will work here.

Refs #6222

Rewrite this method to accept `TomTom::Matrix::Point`s instead of plain
`Geokit::LatLng` coordinates. Do this because this method needs to take
the result of `#points_from_way_costs` as input, and that method now
returns a `Set` of `Point`s.

Refs #6222