Implement IEquatable on structs.

Any struct which overrides object.Equals(object obj) should implement IEquatable<T> to gain a more efficient Equals(T other) overload. This overload will be used by hashing collections like Dictionary which enables them to check equality without boxing the struct.

OpenRA.Game/Input/Hotkey.cs

@@ -9,9 +9,11 @@
  */
 #endregion
 
+using System;
+
 namespace OpenRA
 {
-	public struct Hotkey
+	public struct Hotkey : IEquatable<Hotkey>
 	{
 		public static Hotkey Invalid = new Hotkey(Keycode.UNKNOWN, Modifiers.None);
 		public bool IsValid()
@@ -74,6 +76,11 @@ namespace OpenRA
 
 		public override int GetHashCode() { return Key.GetHashCode() ^ Modifiers.GetHashCode(); }
 
+		public bool Equals(Hotkey other)
+		{
+			return other == this;
+		}
+
 		public override bool Equals(object obj)
 		{
 			var o = obj as Hotkey?;