Support Cargo Workspaces when in RLS Mode

[CryZe]

As far as I can tell (RLS currently doesn't seem to work for me), RLS Mode doesn't support cargo workspaces, so completions only work in the top most crate, which is unfortunate as cargo workspaces are important for larger projects. They work really well in Legacy Mode but seem to be unsupported by the RLS Mode. I may be wrong, as RLS has tons of other issues for me apparently, but if it's actually not something that RLS Mode can do, then it should be fixed. I've talked to other people and it works in other editors that support RLS.

[KalitaAlexey]

It's a duplicate of #158.
Which editor does support workspaces?

[CryZe]

I'm not entirely sure, it was a while ago when somebody told me in the Rust Discord and I don't know who it was. I'll respond to the other issue if I find out. Thanks.

[KalitaAlexey]

Please read my comment in the mentioned issue.
If I am wrong, point me out please.