message: fix compilation after rustc change
diff: use rustc::ty::layout::{Size, HasDataLayout};
  +use rustc::mir::interpret::sign_extend;
  use super::{Scalar, ScalarMaybeUndef, EvalResult};
  -use rustc_mir::interpret::sign_extend;
  pub trait ScalarExt {
  fn null(size: Size) -> Self;
  