message: Add Coerce and CoerceUnsized to mini_core
diff: #[lang="sized"]
  pub trait Sized {}
  +#[lang = "unsize"]
  +pub trait Unsize<T: ?Sized> {}
  +
  +#[lang = "coerce_unsized"]
  +pub trait CoerceUnsized<T> {}
  +
  +impl<'a, 'b: 'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {}
  +
  #[lang="copy"]
  pub unsafe trait Copy {}
  