Having unsolved holes in a 'core' library unneccessarily pollutes the list of holes shown to the user.
Thus, having unfilled holes in a 'core' library is not right.
These constructs can be re-added once the holes have been filled in.
Still a couple of things to resolve in coverage and totality checking
before we can switch on %default, so don't expect quite the right
behaviour just yet. More progress though!
Also working on this has caught a few totality errors in the Idris 2
code base that Idris 1 missed... so these are fixed on the way.
