rustdoc: Avoid using Iterator::count() where possible

`count()` iterates over the whole collection. Using `len()` instead, or
`.next().is_none()` when comparing to zero, should be faster.

src/librustdoc/html/markdown.rs

@@ -656,7 +656,7 @@ impl<'a, I> Footnotes<'a, I> {
     }
 
     fn get_entry(&mut self, key: &str) -> &mut (Vec<Event<'a>>, u16) {
-        let new_id = self.footnotes.keys().count() + 1;
+        let new_id = self.footnotes.len() + 1;
         let key = key.to_owned();
         self.footnotes.entry(key).or_insert((Vec::new(), new_id as u16))
     }

src/librustdoc/passes/unindent_comments.rs

@@ -87,7 +87,7 @@ fn unindent_fragments(docs: &mut Vec<DocFragment>) {
     };
 
     for fragment in docs {
-        if fragment.doc.as_str().lines().count() == 0 {
+        if fragment.doc.as_str().lines().next().is_none() {
             continue;
         }