Make the line numbers of the source code clickable

[barosl]

While talking on IRC, someone wanted to post a link to the Rust source code, but while the lines of the rendered source code do have anchors (<span id="[line number]">), there is no convenient way to make links as they are not clickable. This PR makes them clickable.

Also, a minor fix of the FAQ is included.

[rust-highfive]

r? @huonw

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[steveklabnik]

This change, while good, is totally unrelated

[barosl]

I was in worry of opening another PR to handle such a minor change. (It felt like spamming.) May I open a new one anyways?

[Gankra]

Our actual highlighting script supports ranges: http://doc.rust-lang.org/src/core/slice.rs.html#456-470, which Github's UI supports through shift-clicking line numbers.

Possible?

[barosl]

@gankro Challenge accepted. I've pushed a new implementation. Currently running tests.

[huonw]

This is cool! Would it be possible to publish an example somewhere so we can experiment?

(If you don't have an alternative, you can use github pages: e.g. commit a generated copy to a gh-pages branch in your rust repo and push that, and they'll appear as barosl.github.io/rust/path/to/docs.)

[alexcrichton]

Awesome! Could you also put the new JS wad into main.js instead of inline as well?

[barosl]

@huonw I've uploaded them to https://barosl.com/tmp/rust/doc/src/std/lib.rs.html

@alexcrichton Oops, sorry. I should have considered that. It is fixed now.

[barosl]

I've changed a bit more, using the HTML5 history API(pushState) if the browser supports it. This prevents the page from being scrolled.

Also, I also removed the unused :target style in main.css, which caused color inconsistency when highlighting was triggered by loading a page.

[alexcrichton]

I think this CSS rule was also used for clicking on headers in documentation. That may be covered elsewhere, but can you make sure that if you click on a header that it still gets highlighted?