Show crate names with hyphens in place of underscores

[dtolnay]

Per a comment from @steveklabnik, we would like the crate naming convention to be hyphens instead of underscores. Crates.io should encourage this by showing hyphens in all crate names in the UI.

This should wait until Cargo accepts hyphens: rust-lang/cargo#2775

[dtolnay]

Cargo does not plan to accept hyphens in place of underscores so this won't work.