Enforce keys for all structs

lib/last_fm/album.ex

@@ -1,4 +1,5 @@
 defmodule LastFm.Album do
+  @enforce_keys [:musicbrainz_id, :title]
   defstruct [:musicbrainz_id, :title]
 
   @type t :: %__MODULE__{

lib/last_fm/artist.ex

@@ -1,4 +1,5 @@
 defmodule LastFm.Artist do
+  @enforce_keys [:musicbrainz_id, :name]
   defstruct [:musicbrainz_id, :name, :bio, :image, :play_count]
 
   @type t :: %__MODULE__{

lib/last_fm/config.ex

@@ -8,6 +8,7 @@ defmodule LastFm.Config do
           user_agent: String.t()
         }
 
+  @enforce_keys [:api, :api_key, :user]
   defstruct api: LastFm.Api,
             api_key: "",
             user: "",

lib/last_fm/track.ex

@@ -7,6 +7,15 @@ defmodule LastFm.Track do
 
   alias LastFm.{Album, Artist}
 
+  @enforce_keys [
+    :musicbrainz_id,
+    :title,
+    :artist,
+    :album,
+    :cover_url,
+    :scrobbled_at_uts,
+    :scrobbled_at_label
+  ]
   defstruct [
     :musicbrainz_id,
     :title,