Make prover respect showimplicits from IState
Render using console width
Use the pprint library to insert line breaks

The "Sized" class is not deleted because it is useful in other contexts,
such as eliding parts of error messages.

The "show" instances assume an absurdly wide monitor so as to not break
interactive editing.
Use pretty-printer to display :t for metavars
Merge pull request #827 from david-christiansen/semantic-pprint

Pretty-printer updates