Allow for next-completion to wrap around the completion buffer

Allow for next-completion to wrap around the completion buffer

* lisp/simple.el (completion-wrap-movement): Add new option.
(previous-completion): Update docstring.
(next-completion): Respect completion-wrap-movement.
(switch-to-completions): Handle backwards completion by jumping to the
end of the buffer.
* lisp/minibuffer.el: (minibuffer-local-completion-map): Bind
minibuffer-complete to backtab
(completion--in-region-1): Handle backtab to scroll backwards