rustdoc: Don't search on every single keystroke in the search box

Doing a search can take a full second. This is a major annoyance, especially when it triggers multiple times on mistype + backspace. Also, the results from the first few keystrokes are often not relevant.

Proposed solution: Introduce a delay before starting the search. KDevelop has a similar feature with a default of 500 ms, and it's awesome.

Other solutions:

• speed up the search. A good idea in any case, but it's probably better to simply not do the work in the first place.
• cancel the running search on a new keystroke.
• only trigger on Enter.

[nodakai]

My understanding is, it already has a delay of 100 ms:

• https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/static/main.js#L638

$('.search-input').on('keyup', function() {
    clearTimeout(keyUpTimeout);
    keyUpTimeout = setTimeout(search, 100);
});

You may locally experiment with this parameter.

I also wonder if we should suppress incremental search when the input area has less than, say, three characters.