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 unre-
solved 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 refer-
ence fix. Hopefully the message will be useful to future Rust prog-
rammers.