Consolidate payload generation into new Ziggy class

[bakerkretzmar]

This PR extends and closes #272, extracting all the logic related to generating Ziggy's 'route payload' directly into a new Ziggy class that replaces the existing RoutePayload. It also removes the static compile method in favour of instantiating a new Ziggy object, and resolves the application router instance internally so it doesn't need to be passed in.

The new Ziggy class also implements JsonSerializable, which means we can json_encode() it directly—this makes the use case in the original PR, returning Ziggy's payload as JSON directly from an endpoint, trivial:

return response()->json(new Ziggy('group'));

This PR contains significant breaking changes and will be included in Ziggy 1.0. All code like RoutePayload::compile($router) will need to be replaced with something like (new Ziggy)->toArray() or (new Ziggy)->toArray()['namedRoutes'].

@DanielCoulbourne if you have some time to take a look at this, I'd love to know if it's more or less what you were going for!

[jakebathman]

One change and a question. Good work! Approved for merge after that.

[jakebathman]

Good with this now that tests are all passing 👍