Implement fmt::Debug for std::Command
Created by: MovingtoMars
Used by https://github.com/redox-os/ion/blob/master/src/pipe.rs#L86.
This PR allows the ion submodule to be updated to ion@bb3fe984.
However, ion@474b97e6 breaks the build. Should I make an issue for that on this repo, or the ion repo? (I'm assuming it should be this repo, since ion builds fine using the normal libstd, but fails using the redox libstd)