Changes after std move to library

[tesuji]

The first section of this chapter need to be reword: https://github.com/rust-lang/rustc-dev-guide/blob/master/src/compiler-src.md

The main Rust repository consists of a src directory, under which there live many crates. These crates contain the sources for the standard library and the compiler. This document, of course, focuses on the latter.

Could be rewritten as:
The main Rust repository consists of two directory: src and library, under which there live many crates.
library contains the sources for the standard library.
While src is where compiler and its components live.
This document, of course, focuses on the latter.

[mark-i-m]

Sorry, I've been a bit swamped lately. This is on my list. There is still some change going on, especially wrt to src/librustc_*. #815 fixed a bunch of stuff.