bug#434: Minor Enhancement: Toggle narrowing/widen a function regionand all

[Drew Adams]

> Narrowing to the current function to search around in it
> (especially when refactoring) is something that I often do
> "manually", so having a handy keystroke for it would 
> be nice, I think.
> 
> `C-x n f', for instance, to toggle narrowing to the current function.
> 
> Opinions?

YAGNI.  `narrow-to-defun' is `C-x n d'.  We don't need a toggle for each
narrowing command.  And certainly not another key binding.

This is the kind of thing that any user can do, if desired.  There are lots of
commands that some users might want to bind.  It is trivial for a user to do so.
No need for Emacs to do this by default.  One opinion.