Support `cargo fix` for rename deprecations

[withoutboats]

Ideally, when a deprecation happens because of a rename, cargo fix could rename it for you.

We could make the deprecation warning issue the correct suggestion by adding another field to the rustc_deprecated attribute, as in:

#[rustc_deprecated(renamed = "std::str::str::trim_start")]

[kornelski]

That would be great. I've used these methods in many places.

While I like the new names better, I don't feel the need to change my code, so the deprecation churn is slightly annoying.