Address further review comments

* Use known names for iter/iter_mut method (simplifies checking if the
  method exists
* Extract code to check assist with fixtures to function

crates/hir_expand/src/name.rs

@@ -189,6 +189,7 @@ pub mod known {
         // Components of known path (function name)
         filter_map,
         next,
+        iter_mut,
         // Builtin macros
         file,
         column,