Re-organise cargo workspace

[thomaseizinger]

@Restioson Mentioned in https://github.com/Restioson/xtra/pull/163#pullrequestreview-1068183473 that he'd like a workspace structure where the main xtra crate is moved into its own directory.

I'd suggest that due to the large diff this will cause and the PRs that are still open that we do this as part of the 0.6 release but as the very last step when the dust has settled.