Separate debug/release dependency directories

[mqudsi]

While there are requests (#1359, #1596) to allow for the optimization of dependencies, a suggestion to curb dev times would be to cache the results of dependency builds for debug/release separately, so that switching between cargo build and cargo build --release does not require a full rebuild of all dependencies, only the changed rs source files in question.

[alexcrichton]

This should currently be the case (target/debug and target/release), but is that not working for you?

[mqudsi]

I'm sorry, I've been avoiding switching between them like the plague since I started using rust for this reason; it seems that it has indeed been resolved some time ago and I never dared to try it and find out :)