Remove inline scripts from the rustdoc output

[pietroalbini]

Rustdoc currently emits two inline <script> tags into each HTML page it generates, making it hard to enforce content security policies on websites hosting documentation. The two tags in question are:

<script>window.sidebarCurrent = {name: "rustwide", ty: "mod", relpath: "../"};</script>

<script>window.rootPath = "../";window.currentCrate = "rustwide";</script>

Both of the tags just define variables used by the JavaScript code, and it should be possible to replace them with data- attributes loaded by main.js without other changes to the codebase:

<div id="rustdoc-vars" data-root-path="../" data-current-crate="rustwide" ...></div>

// Top of main.js
var rustdocVars = document.getElementById("rustdoc-vars");
window.rootPath = rustdocVars.attributes["data-root-path"];
window.currentCrate = rustdocVars.attributes["data-current-crate"];