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.
```