the get_byte method needs to take mutable reference to self

the reson for this is it's possible for external hardware to have some
side effect on a read.

src/cpu.rs

@@ -83,9 +83,9 @@ impl<M: Bus> CPU<M> {
                 let x = self.registers.index_x;
                 let y = self.registers.index_y;
 
-                let memory = &self.memory;
+                let memory = &mut self.memory;
 
-                fn read_address<M: Bus>(mem: &M, addr: u16) -> [u8; 2] {
+                fn read_address<M: Bus>(mem: &mut M, addr: u16) -> [u8; 2] {
                     let lo = mem.get_byte(addr);
                     let hi = mem.get_byte(addr.wrapping_add(1));
                     [lo, hi]

src/memory.rs

@@ -59,7 +59,7 @@ impl Default for Memory {
 }
 
 pub trait Bus {
-    fn get_byte(&self, address: u16) -> u8;
+    fn get_byte(&mut self, address: u16) -> u8;
     fn set_byte(&mut self, address: u16, value: u8);
 
     fn set_bytes(&mut self, start: u16, values: &[u8]) {
@@ -78,7 +78,7 @@ impl Memory {
 }
 
 impl Bus for Memory {
-    fn get_byte(&self, address: u16) -> u8 {
+    fn get_byte(&mut self, address: u16) -> u8 {
         self.bytes[address as usize]
     }