Oh, and while you can't select all emails in the Inbox, Apple did give us the ability to Delete All in the Trash and Junk folder in Yahoo, Gmail, and probably most other IMAP services. In iOS 6 you could only do this in an iCloud account.
Swipe back (as per the previous post) then select your account form the Account list, choose the Junk or Trash folder, tap Edit at the top right of the list. You should see a read Delete All option at the bottom right of the list.
Before this all I could recommend was getting the GMail or Yahoo Mail apps and deleting your trash and junk/spam using those apps, or though the webmail interface.
It's these little things I love finding in iOS updates, in many ways more than the big new features.