:unimport functionality for psci

[hdgarrood]

This probably should wait until #817 is resolved.

It would be nice to be able to unimport modules, one at a time. Here's one potential way it could work:

> import Prelude.Unsafe
> :unimport Prelude.Unsafe
> :type unsafeIndex
(error: unknown value `unsafeIndex`)
> import qualified Control.Monad.ST as ST
> import Control.Monad.ST (newSTRef)
> :unimport Control.Monad.ST
Control.Monad.ST has been imported multiple times.
Which import would you like to remove?

   1) import qualified Control.Monad.ST as ST
   2) import Control.Monad.ST (newSTRef)
> 2
Ok, unimporting Control.Monad.ST (newSTRef).
> :type newSTRef
(error: unknown value `newSTRef`)
> :type ST.newSTRef
(works)