Add rustdoc documentation complexity toggle

Adds a simple/detailed toggle to api doc pages.
Detailed mode is the current behaviour, simple mode hides are doccomment details leaving only signatures for quick browsing.
In addition, clicking on a function name now toggles the visibility of the associated doccomment details

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# [Live Build Here](http://cg.scs.carleton.ca/~abeinges/doc/std/vec/struct.Vec.html)

This is something that's been bothering me, and I've seen some people mention in IRC before. The docs are *great* if you want a full indepth look at an API, but *awful* if you want to scan them. This provides the ability to toggle the views freely. Interacts perfectly well with noscript, since "detailed" mode is just what the page always was, and simple mode just hides a bunch of divs.

I'm not much of a designer, so design input welcome on the actual buttons for toggling.

The actual javascript is *a bit* brittle to layout changes, but it always will be without adding lots of extra junk to the actual markup, which didn't seem worth it.
Couldn't this be done in CSS? i.e. add class to some parent element with a rule `.summary-mode .docblock { display: none; }` (this could also be used to control the styling of the buttons).