Building router helpers

One of the most used features of the Phoenix router is the helper functions defined in a router helper module. Phoenix generates this module when a router is compiled. However, to keep our compilation simple, we can decouple router helper modules from the router and define Goldcrest.Router.Helpers, which can be used to define a helper module:

defmodule Goldcrest.Router.Helpers do
  @moduledoc false
  defmacro __using__(opts) do
    quote do
      @opts unquote(opts)
      @default_router_module __MODULE__
                            |> Module.split()
                            ...