Display the target directory in cargo metadata output

[sfackler]

[alexcrichton]

cc @matklad

[matklad]

Should definitely do this! We even need it in IntelliJ Rust. I can implement this, but I don't have a ton of time right now. But it should be pretty strait forward to add: add target_dir to ExportInfo here and populate it from ws.target_dir().

[sfackler]

Done!