Add deprecated LayoutVerified type alias

[joshlf]

Since we renamed LayoutVerified to Ref in #203, we should add a type alias to help ease the transition to 0.7.0. This alias should be marked as #[deprecated] to nudge users in the right direction.

It's also possible that we could use this approach (albeit without the #[deprecated] annotation) to backport the rename to 0.6.x (see #215). Without the deprecation warning, it's unlikely that many people would switch to Ref - the main benefit would be to make future cherry-picks less likely to result in conflights (#203 was a large change that affected many lines of code). We would need to be very careful to ensure that the type alias is genuinely a backwards-compatible standin for the struct itself.