Add an incremental benchmark where only a dependency gets patched

[bjorn3]

In the ideal case the recompilation of the dependent crate would be exactly as fast as a completely unchanged incremental compilation if the dependency didn't change any declarations, generic function definitions or anything of that kind.

[Mark-Simulacrum]

I think we actually have (had?) something like this, but I'd need to track it down. Making it more explicit seems like a good idea, though.