stack, queue: add as.list

It'd be lovely if we were able to convert a stack or a queue to an R list object

so we need

`as.list.stack` and `as.list.queue`

moreover, queue/list constructor should mark `class` of retval to `queue`/`stack`
`print.stack`, `print.queue` and maybe `format.stack` and `format.queue` should be defined

Start by locating the stack and queue constructors and the related methods named in the issue: as.list.stack, as.list.queue, print.stack, print.queue, format.stack, and format.queue. Check how existing constructors assign class and how other data structures are printed or formatted. Done means stack and queue objects convert to R lists and expose consistent class, print, and format behavior.

Written by the indexing model from the issue text.