Add a 'rename parameter' operation for elisp

[Wilfred]

Before:

(defun wh/foo (bar)
  "Do something with BAR."
  (+ bar 1))

After (note the docstring has been updated):

(defun wh/foo (baz)
  "Do something with BAZ."
  (+ baz 1))

[notetiene]

We should be allowed editing the parameter from the function itself. Instead of adding an option, we could allow modifying the docstring if & only if it's a parameter. This could be done by customizing iedit behavior.