Allow logically reborrowing through derefs of ghost and indexing things

[xldenis]

This more correctly implements #836 but the code is suspicious enough that I don't want to merge this yet.

In particular, given x : &mut Ghost<Vec<T>> the expression let y : &mut T = &mut x[0] is expands to: &mut index_logic (* deref (& * x)). The real problem is the insertion of that extra & inside.
My current implementation detects and correctly handles this case for obvious scenarios by 'skipping over it', but is that really what we want to do in general?

[xldenis]

Rewritten since our discussion yesterday @jhjourdan, this turns * Ghost::deref(& _) into a single Ghost::inner(_)