"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 
I think that this needs to use the `RUSTBOOK` variable which notably includes information about `LD_LIBRARY_PATH` and friends. In general this is where I think avoiding this rm/cp/cd chain would be best by teaching rustbook a bit about source roots and such.