rustdoc: CSS for all three themes is fetched

[jsha]

Right now, light1.52.0.css, dark1.52.0.css, and ayu1.52.0.css are all loaded on each page load. These are early in the page, and as high-priority resources, they loaded first. Even with HTTP/2, they block other resources from starting their downloads (because there appears to be a limit on parallel HTTP/2 fetches made by browsers; though I need to check this).

These stylesheets are loaded like:

<link rel="stylesheet" type="text/css" href="../../light1.52.0.css"  id="themeStyle">
<link rel="stylesheet" type="text/css" href="../../dark1.52.0.css" disabled >
<link rel="stylesheet" type="text/css" href="../../ayu1.52.0.css" disabled >

Interestingly, the theme picker JS doesn't change the disabled property, but changes the href on the main theme:

rust/src/librustdoc/html/static/storage.js

Line 111 in 94736c4

var newHref = mainStyleElem.href.replace(fullBasicCss, fullNewTheme);

.

I think it would be more efficient to omit the second two <link>s, and instead list all the theme URLs using a data property on rustdoc-vars.