Add Z80 CPU skeleton, RegisterPair struct, and MemoryBus implementation

Core/Cpu/RegisterPair.cs

@@ -0,0 +1,17 @@
+using System.Runtime.InteropServices;
+
+namespace Core.Cpu
+{
+    [StructLayout(LayoutKind.Explicit)]
+    public struct RegisterPair
+    {
+        [FieldOffset(0)]
+        public ushort Word;
+
+        [FieldOffset(0)]
+        public byte Low;
+
+        [FieldOffset(1)]
+        public byte High;
+    }
+}