Figure out combined Rust+Cargo installation

[brson]

On Unix, the rustup.sh script will install both Rust+Cargo (though that may not end up the official installation mechanism). On Windows, acquiring Cargo is not so convenient.

The obvious thing to do on Windows is to put them both in the same installer, but since Cargo and Rust live in different repos (and should continue to do so for the foreseeable future imo) creating that installer will require some work.

Nominating.

[pnkfelix]

to properly categorize this bug, we need to decide whether cargo is a 1.0-blocking feature.

deferring for a later triage with @alexcrichton present (and hopefully then we can decide about categorizing cargo)

[steveklabnik]

Given that it's the foundation of the new Guide, I would argue that it is. (Or that the guide then needs changed)

[wycats]

I don't see any reason not to consider it a blocker. Cargo largely works already, and the remaining work item, the registry, is well under way.

[pnkfelix]

P-high, 1.0 milestone.

[vadimcn]

So... can we add it as a submodule?