Compilation fixes

Intel8080/Intel8080.cs

@@ -36,7 +36,7 @@ namespace EightBit
 
         public override Register16 HL { get; } = new Register16((int)Mask.Sixteen);
 
-        public override int Execute()
+        public override void Execute()
         {
             var decoded = this.GetDecodedOpCode(this.OpCode);
 
@@ -48,8 +48,6 @@ namespace EightBit
             var q = decoded.Q;
 
             this.Execute(x, y, z, p, q);
-
-            return this.Cycles;
         }
 
         public override int Step()

LR35902/LR35902.cs

@@ -40,7 +40,7 @@ namespace EightBit.GameBoy
 
         private bool Stopped { get; set; } = false;
 
-        public override int Execute()
+        public override void Execute()
         {
             var decoded = this.GetDecodedOpCode(this.OpCode);
 
@@ -61,7 +61,6 @@ namespace EightBit.GameBoy
             }
 
             System.Diagnostics.Debug.Assert(this.Cycles > 0, $"No timing associated with instruction (CB prefixed? {this.prefixCB}) 0x{this.OpCode:X2}");
-            return this.ClockCycles;
         }
 
         public override int Step()

M6502/PageCrossingBehavior.cs

@@ -1,12 +0,0 @@
-// <copyright file="PageCrossingBehavior.cs" company="Adrian Conlon">
-// Copyright (c) Adrian Conlon. All rights reserved.
-// </copyright>
-
-namespace EightBit
-{
-    public enum PageCrossingBehavior
-    {
-        AlwaysReadTwice,
-        MaybeReadTwice,
-    }
-}

MC6809/MC6809.cs

@@ -211,7 +211,7 @@ namespace EightBit
             return this.Cycles;
         }
 
-        public override int Execute()
+        public override void Execute()
         {
             this.LowerBA();
             this.LowerBS();
@@ -232,8 +232,6 @@ namespace EightBit
                     this.Execute11();
                 }
             }
-
-            return this.Cycles;
         }
 
         public void RaiseNMI()

Z80/Z80.cs

@@ -316,7 +316,7 @@ namespace EightBit
             }
         }
 
-        public override int Execute()
+        public override void Execute()
         {
             var decoded = this.GetDecodedOpCode(this.OpCode);
 
@@ -339,8 +339,6 @@ namespace EightBit
             {
                 this.ExecuteOther(x, y, z, p, q);
             }
-
-            return this.Cycles;
         }
 
         public override int Step()