Save-analysis data should indicate whether an item is visible from the crate root

[euclio]

This is needed for tools like the new rustdoc prototype at steveklabnik/rustdoc, where it's required to only document items that can be used by the library consumer.

This should also take pub use into account.

cc rust-dev-tools/rls-analysis#79

[nrc]

cc me

To be clear, I think we should be able to filter data based on whether it is visible, I don't think save-analysis needs to contain that info.

[euclio]

@nrc

I've started working on this. Could you offer any advice around how to correctly handle pub use? I'm thinking something along the lines of: on encountering a pub use, I should additionally see if the used item is reachable. If it is, dump the item even if it's not reachable.