Use iter() for iterating arrays by slice

These `into_iter()` calls will change from iterating references to
values if we ever get `IntoIterator` for arrays, which may break the
code using that iterator. Calling `iter()` is future proof.

src/librustdoc/html/render.rs

@@ -506,7 +506,7 @@ pub fn initial_ids() -> Vec<String> {
      "methods",
      "deref-methods",
      "implementations",
-    ].into_iter().map(|id| (String::from(*id))).collect()
+    ].iter().map(|id| (String::from(*id))).collect()
 }
 
 /// Generates the documentation for `crate` into the directory `dst`