Auto merge of #2759 - alexcrichton:workspaces, r=brson

Auto merge of #2759 - alexcrichton:workspaces, r=brson

Implement workspaces in Cargo

This commit is an implementation of [RFC 1525] which specifies the addition of
**workspaces** to Cargo.

[RFC 1525]: https://github.com/rust-lang/rfcs/blob/master/text/1525-cargo-workspace.md

A workspace is a group of crates which are all compiled into the same output
directory and share the same `Cargo.lock` file. This means that dependencies are
cached between builds as well as dependencies all being shared at the same
versions. An update to any one dependency transitively affects all other members
of the workspace.

Typical repository layouts with a crate at the root and a number of path
dependencies simply need to add the following to the root `Cargo.toml`:

```toml
[workspace]
```

Otherwise more advanced configuration may be necessary through the
`package.workspace` or `workspace.members` keys. More information can be found
as part of [RFC 1525].