rustdoc: use <details> tag for the source code sidebar

This fixes the extremely poor accessibility of the old system, making it
possible to navigate the sidebar by keyboard, and also implicitly gives the
sidebar items the correct ARIA roles.

src/librustdoc/html/static/css/rustdoc.css

@@ -1528,37 +1528,18 @@ kbd {
 	margin-bottom: 1em;
 }
 
-div.children {
-	padding-left: 27px;
-	display: none;
-}
-div.name {
+details.dir-entry > summary {
+	margin: 0 0 0 13px;
+	list-style-position: outside;
 	cursor: pointer;
-	position: relative;
-	margin-left: 16px;
 }
-div.files > a {
+
+details.dir-entry div.folders, details.dir-entry div.files {
+	padding-left: 27px;
+}
+
+details.dir-entry a {
 	display: block;
-	padding: 0 3px;
-}
-div.files > a:hover, div.name:hover {
-	background-color: #a14b4b;
-}
-div.name.expand + .children {
-	display: block;
-}
-div.name::before {
-	content: "\25B6";
-	padding-left: 4px;
-	font-size: 0.625rem;
-	position: absolute;
-	left: -16px;
-	top: 4px;
-}
-div.name.expand::before {
-	transform: rotate(90deg);
-	left: -15px;
-	top: 2px;
 }
 
 /* The hideme class is used on summary tags that contain a span with