Make Creator property Cacheable

Makes the Creator property into an Optional<Cacheable<IUser, ulong>> type.
Checks to see if users are first contained in the cache, and does not
force the end user to download this User.

src/Discord.Net.Core/Entities/Emotes/GuildEmote.cs

@@ -31,14 +31,15 @@ namespace Discord
         /// </returns>
         public IReadOnlyList<ulong> RoleIds { get; }
         /// <summary>
-        ///     Gets the User that created this emoji.
+        ///     Gets the cached User that created this emoji.
         /// </summary>
         /// <returns>
-        ///     An optional <see cref="IUser"/> who created this emoji. An unspecified value only indicates that the creator was not supplied as part of the API response.
+        ///     An optional <see cref="Cacheable{TEntity, TId}"/> <see cref="IUser"/> who created this emoji.
+        ///     An unspecified value only indicates that the creator was not supplied as part of the API response.
         /// </returns>        
-        public Optional<IUser> Creator { get; }
+        public Optional<Cacheable<IUser, ulong>> Creator { get; }
 
-        internal GuildEmote(ulong id, string name, bool animated, bool isManaged, bool requireColons, IReadOnlyList<ulong> roleIds, Optional<IUser> creator) : base(id, name, animated)
+        internal GuildEmote(ulong id, string name, bool animated, bool isManaged, bool requireColons, IReadOnlyList<ulong> roleIds, Optional<Cacheable<IUser, ulong>> creator) : base(id, name, animated)
         {
             IsManaged = isManaged;
             RequireColons = requireColons;

src/Discord.Net.Rest/Extensions/EntityExtensions.cs

@@ -1,5 +1,6 @@
 using System.Collections.Immutable;
 using System.Linq;
+using System.Threading.Tasks;
 
 namespace Discord.Rest
 {
@@ -7,14 +8,19 @@ namespace Discord.Rest
     {
         public static GuildEmote ToEntity(this API.Emoji model, IGuild guild)
         {
-            return new GuildEmote(model.Id.Value, model.Name, model.Animated.GetValueOrDefault(), model.Managed, model.RequireColons, ImmutableArray.Create(model.Roles), GetEmoteAuthor(model, guild));
+            return new GuildEmote(model.Id.Value, model.Name, model.Animated.GetValueOrDefault(), model.Managed, model.RequireColons, ImmutableArray.Create(model.Roles), GetEmoteAuthorAsync(model, guild).Result);
         }
 
-        internal static Optional<IUser> GetEmoteAuthor(API.Emoji model, IGuild guild)
+        internal static async Task<Optional<Cacheable<IUser, ulong>>> GetEmoteAuthorAsync(API.Emoji model, IGuild guild)
         {
             if (!model.User.IsSpecified || guild == null)
-                return new Optional<IUser>();
-            return new Optional<IUser>(guild.GetUserAsync(model.User.Value.Id).Result);
+                // api did not provide the user, or guild provided was null
+                return new Optional<Cacheable<IUser, ulong>>();
+            // get the cached user, if exists in cache
+            var cachedUser = await guild.GetUserAsync(model.User.Value.Id, CacheMode.CacheOnly).ConfigureAwait(false);
+            var isCached = cachedUser != null;
+            var cacheable = new Cacheable<IUser, ulong>(cachedUser, model.User.Value.Id, isCached, async () => await guild.GetUserAsync(model.User.Value.Id).ConfigureAwait(false));
+            return new Optional<Cacheable<IUser, ulong>>(cacheable);
         }
 
         public static Embed ToEntity(this API.Embed model)