Pass `args` to `run` instead of storing it in a field. This avoids the
need to clone it within `run`.
Also, change `args` from `Vec<String>` to `&[String]`, avoiding the need
for some vecs and clones.
When we initially created `CrateItem`, it would only represent items
that contain a body.
That is no longer the case, for now, make this explicit by expanding
the APIs to retrieve the item body.
This is related to https://github.com/rust-lang/project-stable-mir/issues/34
