an option to delete all at one time that I can see.
Me either.
I think the new "mail" app is caught in the development cycle and is missing some features right now.
Not as bad as the new "people" app but still, a work in progress.
Although it's a relatively simple task to open your Trash folder, select the top most item and then just click the hell out of the "delete" item near the top right, of what I'll call for lack of a better term the menu bar, until they're all gone.