Rename Iterator::len to count

This commit carries out the request from issue #14678:

> The method `Iterator::len()` is surprising, as all the other uses of
> `len()` do not consume the value. `len()` would make more sense to be
> called `count()`, but that would collide with the current
> `Iterator::count(|T| -> bool) -> unit` method. That method, however, is
> a bit redundant, and can be easily replaced with
> `iter.filter(|x| x < 5).count()`.
> After this change, we could then define the `len()` method
> on `iter::ExactSize`.

Closes #14678.

[breaking-change]

src/librustdoc/html/toc.rs

@@ -32,7 +32,7 @@ pub struct Toc {
 
 impl Toc {
     fn count_entries_with_level(&self, level: u32) -> uint {
-        self.entries.iter().count(|e| e.level == level)
+        self.entries.iter().filter(|e| e.level == level).count()
     }
 }