Implement Register methods, and stub ADD opcodes

src/bus.rs

@@ -1,14 +1,12 @@
 #[derive(Debug, Copy, Clone)]
-pub struct MemoryBus {
-
-}
+pub struct MemoryBus {}
 
 impl MemoryBus {
-    pub fn read_byte(&self, _address: u16) {
+    pub fn read_byte(&self, _address: u16) -> u8 {
         unimplemented!()
     }
 
     pub fn write_byte(&self, _address: u16) {
         unimplemented!()
     }
-}
+}