add support for "async fn"

Async functions do not show up.   Adding this causes them to show in the imenu.

Author: haroldcarr

rust-mode.el

@@ -1292,7 +1292,7 @@ whichever comes first."
 (defvar rust-imenu-generic-expression
   (append (mapcar #'(lambda (x)
                       (list (capitalize x) (rust-re-item-def-imenu x) 1))
-                  '("enum" "struct" "union" "type" "mod" "fn" "trait" "impl"))
+                  '("async fn" "enum" "struct" "union" "type" "mod" "fn" "trait" "impl"))
           `(("Macro" ,(rust-re-item-def-imenu "macro_rules!") 1)))
   "Value for `imenu-generic-expression' in Rust mode.