Stub Bus, and CPU, implement some opcode decoding.

src/bus.rs

@@ -0,0 +1,20 @@
+#[derive(Debug, Copy, Clone)]
+pub struct MemoryBus {}
+
+impl MemoryBus {
+    pub fn read_byte(&self, _address: u16) -> u8 {
+        unimplemented!()
+    }
+
+    pub fn write_byte(&mut self, _address: u16) {
+        unimplemented!()
+    }
+
+    pub fn read_word(&self, _address: u16) -> u16 {
+        unimplemented!()
+    }
+
+    pub fn write_word(&mut self, __address: u16) {
+        unimplemented!()
+    }
+}