Intro redux

Because my '30 minute intro' was originally a blog post, the tone was a bit too light. It also was written a long time ago, and deserves a bit of a refresher for modern Rust. now that my work on the Guide is wrapping up, I want to give it a quick re-write as well.

This is not yet done, but I'm submitting it for feedback so far. I'd really like some comments on the ownership part in particular, which gets lower level than before, but is not strictly 100% accurate. Trying to strike a balance.

In general, I'm not sure I go into enough detail for those without systems experience, but am afraid of too much detail for those that do.

Rendered view: https://github.com/steveklabnik/rust/blob/intro_redux/src/doc/intro.md

/cc @wycats @nikomatsakis @brson etc
Again, this isn't what Ruby is doing... In general, the Rust `.clone()` will be far more expensive than Ruby, since Ruby is just shuffling pointers, while `.clone` is a "sheep" copy (shallow/deep), theoretically doing enough work to go from a `&T` to a `T`, which can be a lot, e.g. going from a `&Vec<T>` to a `Vec<T>` requires a new allocation and then sheep-copying each element `T` across, this is O(n).

(In this specific case, the use of literals means it's just copying a `&str`, which is just cheaply O(1) shuffling pointers around.)