bug#25929: 25.2; map-delete doesn't delete permanently 1st alist elt

[Stefan Monnier]

>     (macroexp-let2 nil key
>       `(progn
>          (map--dispatch ,map
>            :list (setf (alist-get ,key ,map nil t) nil)
>            :hash-table (remhash ,key ,map)
>            :array (and (>= ,key 0)
>                        (<= ,key (seq-length ,map))
>                        (aset ,map ,key nil)))
>          ,map)))

Note that this will make it pretty much impossible to use
cl-generic dispatch.

A better option might be to provide a map-remove which works
functionally (i.e. doesn't modify its argument by side-effects), and
then change map-delete to signal an error when we ask to delete the
first element of the list (and to stop returning the "new map" since it
just works imperatively instead).


        Stefan