Use cargo workspaces when they stabilize

[rphmeier]

Now that rust-lang/cargo#2759 has landed.

This should improve build times and workflow for multi-crate projects like parity

[rphmeier]

Workspaces have hit nightly (and maybe beta), so I will start work on this in the next release or the following.

[5chdn]

This issue is labelled with sometimesoon: Issue is worth doing soon. But it's neither assigned and nor linked to a milestone.

It probably needs a re-prioritization, or a proper deadline and an assignee.