Implementing 'rustpkg init'.

This will initialize a new workspace.
auto merge of #9236 : steveklabnik/rust/rustpkg_init, r=catamorphism

Closes #9045

Built on top of #9235, which isn't strictly needed for now, but I imagine I will use part of. Unsure.

I mostly wanted to start this off to get some feedback from @catamorphism and others. These are the directories that actually need made, but I was thinking about adding a few other things:

1. an `examples` directory, since it seems like that's a common pattern
2. a `.gitignore` file that ignores `build`. And anything else that makes sense
3. a sample module that'd actually compile


Feedback?