bug#13343: 24.3.50; give examples using `gv-define-expander' and `gv-letplace'

[Lars Ingebrigtsen]

"Drew Adams" <drew.adams@oracle.com> writes:

> (elisp) `Adding Generalized Variables' says only this:
>
>   For more control over the expansion, see the macro `gv-define-expander'.
>   The macro `gv-letplace' can be useful in defining macros that perform
>   similarly to `setf'; for example, the `incf' macro of Common Lisp.
>   Consult the source file `gv.el' for more details.
>
> That's a complete cop-out.  Give at least one example of using each of
> `gv-define-expander' and `gv-letplace'.

I've now added this to the trunk.  

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