auto merge of #5014 : Kimundi/rust/incoming, r=graydon

This implements a rust tool similar to go's go tool. It acts as a central hub for the tools rustc, rustdoc, rustpkg, rusti, and also provides shortcuts for directly compiling and running a source file either as a normal or a test executable. Default usage message:

```
The rust tool is a convenience for managing rust source code.
It acts as a shortcut for programs of the rust tool chain.

Usage:	rust <command> [arguments]

The commands are:

    build   compile rust source files
    run     build a executable, and run it
    test    build a test executable, and run it
    doc     generate documentation from doc comments
    pkg     download, build, install rust packages
    sketch  run a rust interpreter
    help    show detailed usage of a command

Use "rust help <command>" for more information about a command.
```