Make a dedicated function for advancing the PC by one instruction

src/core.rs

@@ -92,4 +92,8 @@ impl Core {
         }
         self.x_regs[id as usize] = value;
     }
+
+    pub(crate) fn advance_pc(&mut self) {
+        self.pc = self.pc.wrapping_add(4);
+    }
 }