Add a way to only optimize dependencies

[bjorn3]

I have a project which has a test taking 30 sec in debug mode and 1 sec in release mode. I think most of the time is spent in a dependency, so I want to have it optimized, but I also don't want my project itself to be optimized, because it takes much time to compile it.

[alexcrichton]

This is indeed something we'd like to have! This is implemented on nightly and has a separate tracking issue, so I'm going to close in favor of those.