Fix #995, Move Category Implementation from IGuildChannel to INestedChannel (#1004)

* Fix #995 ICategoryChannel.CategoryID throws NotSupportedException

* Add tests

* change run mode of TestChannelCategories

* Add throw for GetCategoryAsync

* Add xml doc explaining why exception is thrown

* Add test coverage for text and voice channel categories

* initial implementation of INestedChannel

* more implementation of INestedChannel design

* Add case in RestChannel Create for Category type

* set the CategoryID for RestVoiceChannel

* rewrite channel category tests to work with existing pattern

* remove outdated todo

* Make IVoiceChannel implement INestedChannel

* remove redundant interface implementation

* Add c#7 feature from feedback

* Remove redundant GetCategoryAsync methods from socket entities

* Added configureawait to async methods

* change signature of interface GetCategoryAsync

* Add check for cachemode in rest channel GetCategory

* remove redundant IGuildChannel interface from ITextChannel and IVoiceChannel

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -316,6 +316,16 @@ namespace Discord.Rest
             return models.Select(x => RestWebhook.Create(client, channel, x))
                 .ToImmutableArray();
         }
+        // Categories
+        public static async Task<ICategoryChannel> GetCategoryAsync(INestedChannel channel, BaseDiscordClient client, RequestOptions options)
+        {
+            // if no category id specified, return null
+            if (!channel.CategoryId.HasValue)
+                return null;
+            // CategoryId will contain a value here
+            var model = await client.ApiClient.GetChannelAsync(channel.CategoryId.Value, options).ConfigureAwait(false);
+            return RestCategoryChannel.Create(client, model) as ICategoryChannel;
+        }
 
         //Helpers
         private static IUser GetAuthor(BaseDiscordClient client, IGuild guild, UserModel model, ulong? webhookId)