New guide: crates and modules

This is an almost-done draft of a guide on crates and modules. This is a hard guide to get right, I had to remove a chunk of the Guide because it was confusing.

I've also pushed up https://github.com/steveklabnik/phrases which has matching code. Whenever we finish this guide, I think it'd be good to have a sample crate like this in the rust-lang org for people to compare against. The hardest part of a guide like this is that it depends on multiple files being correct, and being able to point to a repository would be very helpful.

Things yet to do:

1. external crates via cargo
2. documentation

I'm super open to still revising this if it's still confusing. There's been a lot of Reddit discussion about the module system, and I tried to incorporate those posts and the comments into this.
It doesn't look like we [currently have a guideline](http://aturon.github.io/style/imports.html) for this. cc @aturon, this may be a good thing to have a guideline on (multi-line imports).

The current rustc codebase tends to do:

```rust
use foo::bar::{...};
use foo::bar::{...};
use foo::bar::{...};
```

Whereas cargo (which I've been personally moving towards rustc style), and perhaps some other projects do:

```rust
use foo::bar::{
    ...,
    ...,
};
```