Rollup merge of #44989 - QuietMisdreavus:what-is-your-quest, r=GuillaumeGomez

let rustdoc print the crate version into docs

This PR adds a new unstable flag to rustdoc, `--crate-version`, which when present will add a new entry to the sidebar of the root module, printing the given version number:

![Screenshot of a test crate, showing "Version 1.3.37" under the crate name](https://user-images.githubusercontent.com/5217170/31104096-805e3f4c-a7a0-11e7-96fc-368b6fe063d6.png)

Closes #24336

(The WIP status is because i don't want to merge this until i can get the std docs to use it, which i need help from rustbuild people to make sure i get right.)

src/librustdoc/clean/mod.rs

@@ -112,6 +112,7 @@ impl<T: Clean<U>, U> Clean<Vec<U>> for P<[T]> {
 #[derive(Clone, Debug)]
 pub struct Crate {
     pub name: String,
+    pub version: Option<String>,
     pub src: PathBuf,
     pub module: Option<Item>,
     pub externs: Vec<(CrateNum, ExternalCrate)>,
@@ -183,6 +184,7 @@ impl<'a, 'tcx> Clean<Crate> for visit_ast::RustdocVisitor<'a, 'tcx> {
 
         Crate {
             name,
+            version: None,
             src,
             module: Some(module),
             externs,