"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 
Yeah that's just wrong 😅

> On Dec 18, 2014, at 21:20, Alex Crichton <notifications@github.com> wrote:
> 
> In src/rustbook/target/.fingerprint/rustbook-ef15939ef756fdd3/dep-bin-rustbook:
> 
> > @@ -0,0 +1,2 @@
> > +/home/steve/src/rust/src/rustbook/target/rustbook: /home/steve/src/rust/src/rustbook/src/main.rs /home/steve/src/rust/src/rustbook/src/term.rs /home/steve/src/rust/src/rustbook/src/error.rs /home/steve/src/rust/src/rustbook/src/book.rs /home/steve/src/rust/src/rustbook/src/subcommand.rs /home/steve/src/rust/src/rustbook/src/help.rs /home/steve/src/rust/src/rustbook/src/build.rs /home/steve/src/rust/src/rustbook/src/serve.rs /home/steve/src/rust/src/rustbook/src/test.rs /home/steve/src/rust/src/rustbook/src/css.rs
> > +
> This looks like you're checking in some generated files?
> 
> —
> Reply to this email directly or view it on GitHub.
>