Guide: intro edits

This is mostly organizational and simplifying. However, I also removed prose which I took to be too basic given that we expect this to be read by programmers. Comments welcome.

cc @steveklabnik 
Fair enough, but I don't think that's true -- most Rust I've encountered in the wild is written largely in an expression style and also uses struct initializers, splits long function or method or macro argument lists across several lines.