Simplify some expressions by using the assign op pattern.

See also https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#assign_op_pattern

src/cpu.rs

@@ -448,7 +448,7 @@ impl CPU {
     fn shift_right_with_flags(p_val: &mut u8, status: &mut Status) {
         let mask = 1;
         let is_bit_0_set = (*p_val & mask) == mask;
-        *p_val = *p_val >> 1;
+        *p_val >>= 1;
         status.set_with_mask(
             PS_CARRY,
             Status::new(StatusArgs {

src/registers.rs

@@ -100,39 +100,39 @@ impl Status {
         let mut out = Status::empty();
 
         if negative {
-            out = out | PS_NEGATIVE
+            out |= PS_NEGATIVE
         }
         if overflow {
-            out = out | PS_OVERFLOW
+            out |= PS_OVERFLOW
         }
         if unused {
-            out = out | PS_UNUSED
+            out |= PS_UNUSED
         }
         if brk {
-            out = out | PS_BRK
+            out |= PS_BRK
         }
         if decimal_mode {
-            out = out | PS_DECIMAL_MODE
+            out |= PS_DECIMAL_MODE
         }
         if disable_interrupts {
-            out = out | PS_DISABLE_INTERRUPTS
+            out |= PS_DISABLE_INTERRUPTS
         }
         if zero {
-            out = out | PS_ZERO
+            out |= PS_ZERO
         }
         if carry {
-            out = out | PS_CARRY
+            out |= PS_CARRY
         }
 
         out
     }
 
     pub fn and(&mut self, rhs: Status) {
-        *self = *self & rhs;
+        *self &= rhs;
     }
 
     pub fn or(&mut self, rhs: Status) {
-        *self = *self | rhs;
+        *self |= rhs;
     }
 
     pub fn set_with_mask(&mut self, mask: Status, rhs: Status) {