Added :measure-debug, analogue of :guard-debug for termination proofs.

Quoting :doc note-7-0:

  A new [xargs] keyword, :[measure-debug], decorates each termination
  proof goal with [extra-info] calls that show the source of the
  goal. Thanks to Jared Davis (first in 2008!), Sol Swords, and Anna
  Slobodova for requesting this feature.