Add UI to diff across crates

[xfbs]

diff.rs can already do a diff between crates, for example diff mycrate 0.1.0 and mycrate-fork 0.1.0. However, this is currently not expose in the UI (there is no way in the UI to do this). The only way is to change the URL manually, like /mycrate/0.1.0/mycrate-fork/0.1.0/Cargo.toml. There should be some way in the UI to do this.

My suggestion would be to make the name of the crate (in the UI) editable. If you click on it, it transforms into an input element, and you can manually write a crate name. Not sure if this is the most obvious way to do it.