"The Rust Programming Language," Take II

An updated version of https://github.com/rust-lang/rust/pull/19461

This version vendors aturon/rust-book@731f7bf and builds it when building the docs. This is almost great, except my `make`-foo is poor, so I have my own personal paths in `mk/docs.mk`. How should I best get around that?

/cc @brson 
Changing it to that leads to 

```
/bin/sh: 1: ../../LD_LIBRARY_PATH=/home/steve/src/rust/x86_64-unknown-linux-gnu/stage2/lib:/home/steve/lib:: not found
/bin/sh: 1: ../../LD_LIBRARY_PATH=/home/steve/src/rust/x86_64-unknown-linux-gnu/stage2/lib:/home/steve/lib:: not found
```