bug#66509: 29.1.50; let-alist should support numeric indexing

[Spencer Baugh]

It would be nice if let-alist supported indexing.  So if my alist
contained lists in some places, instead of writing:

(let-alist alist
  (let-alist (nth 0 .a)
    (let-alist (nth 3 .b)
       .c)))

I could instead write:

(let-alist alist
  .a.0.b.3.c)

A patch which does this to follow.