Implemented atomic add/remove role endpoints

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -387,6 +387,26 @@ namespace Discord.API
                     break;
             }
         }
+        public async Task AddRoleAsync(ulong guildId, ulong userId, ulong roleId, RequestOptions options = null)
+        {
+            Preconditions.NotEqual(guildId, 0, nameof(guildId));
+            Preconditions.NotEqual(userId, 0, nameof(userId));
+            Preconditions.NotEqual(roleId, 0, nameof(roleId));
+            options = RequestOptions.CreateOrClone(options);
+
+            var ids = new BucketIds(guildId: guildId);
+            await SendAsync("PUT", () => $"guilds/{guildId}/members/{userId}/roles/{roleId}", ids, options: options);
+        }
+        public async Task RemoveRoleAsync(ulong guildId, ulong userId, ulong roleId, RequestOptions options = null)
+        {
+            Preconditions.NotEqual(guildId, 0, nameof(guildId));
+            Preconditions.NotEqual(userId, 0, nameof(userId));
+            Preconditions.NotEqual(roleId, 0, nameof(roleId));
+            options = RequestOptions.CreateOrClone(options);
+
+            var ids = new BucketIds(guildId: guildId);
+            await SendAsync("DELETE", () => $"guilds/{guildId}/members/{userId}/roles/{roleId}", ids, options: options);
+        }
         
         //Channel Messages
         public async Task<Message> GetChannelMessageAsync(ulong channelId, ulong messageId, RequestOptions options = null)

src/Discord.Net.Rest/Entities/Users/RestGuildUser.cs

@@ -84,10 +84,10 @@ namespace Discord.Rest
         }
         /// <inheritdoc />
         public Task AddRolesAsync(IEnumerable<IRole> roles, RequestOptions options = null)
-            => UserHelper.ModifyAsync(this, Discord, x => x.RoleIds = new Optional<IEnumerable<ulong>>(_roleIds.Concat(roles.Select(xr => xr.Id))), options);
+            => UserHelper.AddRolesAsync(this, this.Discord, roles, options);
         /// <inheritdoc />
         public Task RemoveRolesAsync(IEnumerable<IRole> roles, RequestOptions options = null)
-            => UserHelper.ModifyAsync(this, Discord, x => x.RoleIds = new Optional<IEnumerable<ulong>>(_roleIds.Except(roles.Select(xr => xr.Id))), options);
+            => UserHelper.RemoveRolesAsync(this, this.Discord, roles, options);
         public Task KickAsync(RequestOptions options = null)
             => UserHelper.KickAsync(this, Discord, options);
 

src/Discord.Net.Rest/Entities/Users/UserHelper.cs

@@ -1,5 +1,6 @@
 using Discord.API.Rest;
 using System;
+using System.Collections.Generic;
 using System.Threading.Tasks;
 using Model = Discord.API.User;
 using ImageModel = Discord.API.Image;
@@ -63,5 +64,17 @@ namespace Discord.Rest
             var args = new CreateDMChannelParams(user.Id);
             return RestDMChannel.Create(client, await client.ApiClient.CreateDMChannelAsync(args, options).ConfigureAwait(false));
         }
+
+        public static async Task AddRolesAsync(IGuildUser user, BaseDiscordClient client, IEnumerable<IRole> roles, RequestOptions options)
+        {
+            foreach (var role in roles)
+                await client.ApiClient.AddRoleAsync(user.Guild.Id, user.Id, role.Id, options);
+        }
+
+        public static async Task RemoveRolesAsync(IGuildUser user, BaseDiscordClient client, IEnumerable<IRole> roles, RequestOptions options)
+        {
+            foreach (var role in roles)
+                await client.ApiClient.RemoveRoleAsync(user.Guild.Id, user.Id, role.Id, options);
+        }
     }
 }

src/Discord.Net.WebSocket/Entities/Users/SocketGuildUser.cs

@@ -109,10 +109,10 @@ namespace Discord.WebSocket
             => UserHelper.KickAsync(this, Discord, options);
         /// <inheritdoc />
         public Task AddRolesAsync(IEnumerable<IRole> roles, RequestOptions options = null)
-            => UserHelper.ModifyAsync(this, Discord, x => x.RoleIds = new Optional<IEnumerable<ulong>>(_roleIds.Concat(roles.Select(xr => xr.Id))), options);
+            => UserHelper.AddRolesAsync(this, this.Discord, roles, options);
         /// <inheritdoc />
         public Task RemoveRolesAsync(IEnumerable<IRole> roles, RequestOptions options = null)
-            => UserHelper.ModifyAsync(this, Discord, x => x.RoleIds = new Optional<IEnumerable<ulong>>(_roleIds.Except(roles.Select(xr => xr.Id))), options);
+            => UserHelper.RemoveRolesAsync(this, this.Discord, roles, options);
 
         public ChannelPermissions GetPermissions(IGuildChannel channel)
             => new ChannelPermissions(Permissions.ResolveChannel(Guild, this, channel, GuildPermissions.RawValue));