Comparers (#929)

* Add entity equality comparers

* Fix namespace #whoops

* Add Message comparer.

* Add comment explaining the specialized implementation

* Remove specialized implementation, as per feedback

src/Discord.Net.Core/Utils/Comparers.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+
+namespace Discord
+{
+    public static class DiscordComparers
+    {
+        // TODO: simplify with '??=' slated for C# 8.0
+        public static IEqualityComparer<IUser> UserComparer => _userComparer ?? (_userComparer = new EntityEqualityComparer<IUser, ulong>());
+        public static IEqualityComparer<IGuild> GuildComparer => _guildComparer ?? (_guildComparer = new EntityEqualityComparer<IGuild, ulong>());
+        public static IEqualityComparer<IChannel> ChannelComparer => _channelComparer ?? (_channelComparer = new EntityEqualityComparer<IChannel, ulong>());
+        public static IEqualityComparer<IRole> RoleComparer => _roleComparer ?? (_roleComparer = new EntityEqualityComparer<IRole, ulong>());
+        public static IEqualityComparer<IMessage> MessageComparer => _messageComparer ?? (_messageComparer = new EntityEqualityComparer<IMessage, ulong>());
+
+        private static IEqualityComparer<IUser> _userComparer;
+        private static IEqualityComparer<IGuild> _guildComparer;
+        private static IEqualityComparer<IChannel> _channelComparer;
+        private static IEqualityComparer<IRole> _roleComparer;
+        private static IEqualityComparer<IMessage> _messageComparer;
+
+        private sealed class EntityEqualityComparer<TEntity, TId> : EqualityComparer<TEntity>
+            where TEntity : IEntity<TId>
+            where TId : IEquatable<TId>
+        {
+            public override bool Equals(TEntity x, TEntity y)
+            {
+                bool xNull = x == null;
+                bool yNull = y == null;
+
+                if (xNull && yNull)
+                    return true;
+
+                if (xNull ^ yNull)
+                    return false;
+
+                return x.Id.Equals(y.Id);
+            }
+
+            public override int GetHashCode(TEntity obj)
+            {
+                return obj?.Id.GetHashCode() ?? 0;
+            }
+        }
+    }
+}