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
It's not just not getting the 'correct answer' (since that's rather nebuous, e.g. Rust doesn't protect against non-data-race race conditions which also give incorrect answers), it's specifically the risk of getting data corruption, since a data structure can be read halfway through a write, leaving the reader with an inconsistent/invalid state

e.g. concurrently mutating a `Vec<T>` via `push` and `pop` (i.e. changing the length) can cause a reader to read halfway through a push, e.g. after the length is updated but before the element is actually written, leaving that last slot invalid.