This PR converts the route params array into an object and moves the
error generation code into its own object as well. This is a refactoring
of internal code to make it easier to change and simplier for internals
to interface with. We have two new objects:

1) `RouteWithParams`

Previously `#generate` returned an array of parameterized parts for a
route. We have now turned this into an object with a `path` and a
`params methods` to be able to access the parts we need.

2) `MissingRoute`

`#generate` was also responsible for constructing the message for the
error. We've moved this code into the new `MissingRoute` object so it
does that work instead.

This change makes these methods reusable throughout the code and easier
for future refactoring we plan to do.

Co-authored-by: Aaron Patterson <aaron.patterson@gmail.com>