stable_mir: Add `MutMirVisitor` Resolves: [rust-lang/project-stable-mir#81](https://github.com/rust-lang/project-stable-mir/issues/81). I am unsure if we should add a `MutableBody` like Kani did. Currently, I use `&mut Body` in `MutMirVisitor::visit_body()`. r? ``````@celinval``````