Support Cargo workspaces

[Boddlnagg]

See http://doc.crates.io/manifest.html#the-workspace-section

Difficulties include:

• The target folder might not be where Cargo.toml is (important for locating exe for debugging)
• What should we do when a Cargo.toml is opened (as a new project) that only contains a [workspace] section?
• ... probably more ...