rustdoc: do not allocate String when writing path full name

No idea if this makes any perf difference, but it just seems like premature
pessimisation to use String when str will do.

src/librustdoc/clean/types.rs

@@ -2175,8 +2175,8 @@ impl Path {
     pub(crate) fn whole_name(&self) -> String {
         self.segments
             .iter()
-            .map(|s| if s.name == kw::PathRoot { String::new() } else { s.name.to_string() })
-            .intersperse("::".into())
+            .map(|s| if s.name == kw::PathRoot { "" } else { s.name.as_str() })
+            .intersperse("::")
             .collect()
     }