Implement pretty-printing of imports

[mrkkrp]

Close #27.

This should be ready.

[mrkkrp]

Actually I found a decent way to sort explicit import lists too. Some bits of AST are not clear, but we can adjust the code when we discover concrete cases where they are used.

[facundominguez]

Will we preserve line breaks in import lists?

import Package.Internal.A
import Package.Internal.B

import External.A
import External.B

I've added some tests which fail.

Also, we need to advice on how to deal with a redundant Prelude:

{-# OPTIONS_GHC -Wall -Werror #-}
import Text.Read
import Prelude

f :: Int
f = read "0"

The above program would fail to build in GHC if imports are reordered.

{-# OPTIONS_GHC -Wall -Werror #-}
import Prelude
import Text.Read

f :: Int
f = read "0"

error: [-Wunused-imports, -Werror=unused-imports]
    The import of 'Text.Read' is redundant
      except perhaps to import instances from 'Text.Read'
    To import instances alone, use: import Text.Read()
  |
3 | import Text.Read
  | ^^^^^^^^^^^^^^^^

[mrkkrp]

I'm going to merge this one, Prelude is always placed last in the import list, this should be OK in all cases. For dropping of comments that we do I'm going to open another issue so we won't forget about the problem.