Allow nested destructuring with key->key

[Fuco1]

(-let (((&plist :foo->:bar value) (list :foo (list :bar "x"))))
  (message "value is %s" value))

;; should also work like this to support "string"->"keys"
(-let (((&plist :foo -> :bar value) (list :foo (list :bar "x"))))
  (message "value is %s" value)

We can also think about a different delimiter, but not . because that is the cons read syntax.