bug#49407: Request: Specify default values in `map-let` in Map.el

[Okam]

Hello,

`map-let` allows one to conveniently bind variables using `map-elt`, but
does not provide a way specify a default value if a key is missing.

With `map-elt`, one can use the optional third argument to specify this
value.  It would be good to have this in `map-let` as well.

For example, maybe it could look something like

     ;; As just a third value in the list:
     (let ((map '(:a 1 :b 2)))
       (map-let ((:a a)
                 (:b b)
                 (:c c 3))
           map
         (+ a b c)))

or

     ;; More like Common Lisp arg-list for `&key'.
     (let ((map '(:a 1 :b 2)))
       (map-let ((:a a)
                 (:b b)
                 (:c (c 3)))
           map
         (+ a b c)))

Please consider adding this feature.

Thank you.