Auto merge of #38414 - estebank:doc-dissambiguate, r=steveklabnik

Rustdoc: disambiguate Implementors when the type name is not unique

Presentation [goes from](https://doc.rust-lang.org/stable/std/iter/trait.ExactSizeIterator.html#implementors):

<img width="492" alt="" src="https://cloud.githubusercontent.com/assets/1606434/21276752/b2b50474-c387-11e6-96e1-9766851da269.png">

to:

<img width="787" alt="" src="https://cloud.githubusercontent.com/assets/1606434/21276763/bb37f6b0-c387-11e6-8596-9163cb254674.png">

on cases where there're multiple implementors with the same name.

Fixes #37762.

Author: bors

src/librustdoc/clean/mod.rs

@@ -2219,8 +2219,8 @@ impl Path {
         }
     }
 
-    pub fn last_name(&self) -> String {
-        self.segments.last().unwrap().name.clone()
+    pub fn last_name(&self) -> &str {
+        self.segments.last().unwrap().name.as_str()
     }
 }