Redesign / rewrite rustdoc

[brson]

Rustdoc was implemented for Rust 0.1 and a lot about the language has changed since then. Both the way it presents certain features and the way it's implemented are relics. It's never been particularly good and probably deserves a comprehensive redesign.

[graydon]

accepted for backwards-compatible milestone

[graydon]

(because this will likely change command-line interface to the tool)

[emberian]

I'm actively doing this

[emberian]

bam, done. yay!

[emberian]

#8125

[emberian]

Now that rustdoc_ng has been merged, closing.