Lots of improvements to rustdoc

The commit messages are a good technical summary, a good visual summary (contrib is this version):

Pub use statements now rendered. Notice how almost all components are also clickable!
* http://static.rust-lang.org/doc/master/std/prelude/index.html
* http://www.contrib.andrew.cmu.edu/~acrichto/doc/std/prelude/index.html

Private things hidden by default (for at least some approximation of privacy). I hope to improve this once privacy is totally ironed out.
* http://static.rust-lang.org/doc/master/std/hashmap/struct.HashMap.html
* http://www.contrib.andrew.cmu.edu/~acrichto/doc/std/hashmap/struct.HashMap.html

Unindentation now works properly:
* http://static.rust-lang.org/doc/master/extra/getopts/index.html
* http://www.contrib.andrew.cmu.edu/~acrichto/doc/extra/getopts/index.html

Also sundown has massively reduced compilation time (of docs, not the of the crates)