rustdoc: Provide a general --default-setting SETTING[=VALUE] option

We just plumb through what the user tells us.

This is flagged as unstable, mostly because I don't understand the
compatibility rules that rustdoc obeys for local storage data, and how
error handling of invalid data works.

We collect() the needed HashMap from Vec of Vecs of (key, value)
pairs, so that there is a nice place to add new more-specific options.
It would have been possible to use Extend::extend but doing it this
way ensures that all the used inputs are (and will stay) right next to
each other.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

src/librustdoc/html/layout.rs

@@ -178,7 +178,7 @@ pub fn render<T: Print, S: Print>(
         default_settings = layout
             .default_settings
             .iter()
-            .map(|(k, v)| format!(r#" data-{}="{}""#, k.replace('-',"_"), Escape(v),))
+            .map(|(k, v)| format!(r#" data-{}="{}""#, k.replace('-', "_"), Escape(v),))
             .collect::<String>(),
         style_files = style_files
             .iter()