Import entities from Control.Monad explicitly since mtl-2.3 stops re-exporting Control.Monad

[msakai]

I noticed that ersatz failed to compile with mtl-2.3 with the following errors:

ersatz/examples/regexp-grid/RegexpGrid/Problem.hs:134:19: error:
    Variable not in scope: guard :: Bool -> m b
    |
134 |     endOfFields = guard . Seq.null =<< use rbsFields
    |                   ^^^^^

ersatz/examples/sudoku/Sudoku/Problem.hs:27:17: error:
    • Variable not in scope: replicateM :: Int -> m2 a1 -> m [Cell]
    • Perhaps you meant ‘replicate’ (imported from Prelude)
   |
27 |             <$> replicateM (Array.rangeSize range) exists
   |                 ^^^^^^^^^^

ersatz/examples/sudoku/Sudoku/Problem.hs:32:3: error:
    Variable not in scope:
      forM_ :: [(Index, Word8)] -> ((Index, Word8) -> t1) -> m a0
   |
32 |   forM_ (Array.assocs initValues) $ \(idx, val) ->
   |   ^^^^^

ersatz/examples/sudoku/Sudoku/Problem.hs:33:5: error:
    Variable not in scope: when :: Bool -> m1 () -> t1
   |
33 |     when (1 <= val && val <= 9) $
   |     ^^^^

ersatz/examples/sudoku/Sudoku/Problem.hs:58:3: error:
    Variable not in scope:
      forM_ :: [(Cell, Cell)] -> ((t0, t0) -> m0 ()) -> ReaderT Env m ()
   |
58 |   forM_ pairs $ \(cellA, cellB) -> assert (cellA /== cellB)
   |   ^^^^^

This is because mtl-2.3 stopped re-exporing Control.Monad from Control.Monad.Reader so that importing Control.Monad.Reader is not enough for using those functions.

Note that mtl-2.3 was deprecated, so there may be no need to rush to fix the issue.