message: Fix biguint assert_assign_op for rust 1.8
  The `$op` only seems to be accepted when used indirectly through another
  macro, in this case `assert_eq`.  How nice.
diff: @@ -29,11 +29,10 @@ macro_rules! assert_assign_op {
  ($left:ident $op:tt $right:ident == $expected:expr) => {
  {
  let mut tmp12384 = $left.clone();
  - tmp12384 $op &$right;
  - assert_eq!(tmp12384, $expected);
  - tmp12384 = $left.clone();
  - tmp12384 $op $right.clone();
  - assert_eq!(tmp12384, $expected);
  + assert_eq!({ tmp12384 $op &$right; tmp12384}, $expected);
  +
  + let mut tmp12384 = $left.clone();
  + assert_eq!({ tmp12384 $op $right.clone(); tmp12384}, $expected);
  }
  };
  }
  