Extracting response helpers

First, let’s move the render functions in Goldcrest.View to a helper module, which can also be imported into the Goldcrest.Controller.__using__/1 macro, allowing the controller to use functions such as redirect and render:

defmodule Goldcrest.ResponseHelpers do
  @moduledoc false
  import Plug.Conn
  require EEx
  def render(conn, :json, data) when is_map(data) do
    conn
    |> content_type("application/json")
    |> send_resp(200, Jason.encode!(data))
  end
  def render(conn, :json, data) when is_binary(data) do
    data = ensure_json!(data)
    conn
    |> content_type("application/json")
    |> send_resp(conn.status || 200, data)
  end
  def render(conn, :html, data) when is_binary(data) do
...