Initial split of the modules

NadekoBot.Core/_Extensions/IEnumerableExtensions.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace NadekoBot.Extensions
+{
+    public static class IEnumerableExtensions
+    {
+        public static IEnumerable<T> Distinct<T, U>(this IEnumerable<T> data, Func<T, U> getKey) =>
+            data.GroupBy(x => getKey(x))
+                .Select(x => x.First());
+    }
+}