Suggest possible fix for missing library link

I'm attempting to learn to write programs using Rust. One problem I encountered early on was errors about failed imports due to unresolved names. The problem was I overlooked the need to declare the link to `std` before importing names into my module.

I thought `rustc` could use a more helpful diagnostic message when it encounters this issue, so I went ahead and implemented a reference fix. Hopefully the message will be useful to future Rust programmers.