Use pretty-printer for :i command

This enables proper semantic highlighting in IDESlave and fixes the
specific problems in issue #1029.
Properly show superclass constraints in :i

Before:
    (a : Type) -> (Monoid a) -> (Eq a) -> Cast a (Maybe a)

Now:
    Monoid a => Eq a => Cast a (Maybe a)

Fixes #1029.
Show type class parameters in :i

Also, slightly more readable formatting.
Merge pull request #1039 from david-christiansen/issue/1029

Use pretty-printer for :i command