Guide: Hello, world!

Let's write our first Rust program!
auto merge of #15181 : steveklabnik/rust/hello_world, r=brson

This is built on top of https://github.com/rust-lang/rust/pull/15162 . https://github.com/steveklabnik/rust/commit/cccae83d922f5aade2b7cca96579521cc35ced59 is the only new commit, you may want to look at that rather than the whole diff.

Writing our first Rust program together. This is the most crucial step, so I go to a fairly deep level of detail. Future sections will move more quickly.