Add cl- version of async-defun and async-lambda

[isamert]

It would be nice to have async-cl-defun, I defined the following and it works for me:

;;;###autoload
(defmacro async-cl-defun (name arglist &rest body)
  "Same as `async-defun' but uses `cl-defun' to define the function."
  (declare (doc-string 3) (indent 2))
  (cl-assert lexical-binding)
  (let* ((parsed-body (macroexp-parse-body body))
         (declarations (car parsed-body))
         (exps (macroexpand-all
                `(cl-macrolet
                     ((await (value)
                             `(async-await--check-return-value (iter-yield ,value))))
                   ,@(cdr parsed-body))
                macroexpand-all-environment)))
    `(cl-defun ,name ,arglist
       ,@declarations
       (async-await--awaiter
        (funcall (iter2-lambda () ,exps))))))

If you are okay adding this, I can put together a PR (instead of using the implementation above, I will try to create one without doing code duplication).