Fix compilation with GHC 9.0 (#40)

See https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0#simplified-subsumption
for the required eta-expansion in lensWithDefault'

Hiding singleton from BasePrelude gives a warning for previous GHC versions,
but this is a minor issue

src/Util.hs

@@ -80,7 +80,7 @@ lensWithDefault guess setter getter f x =
     (\y → setter (Just y) x) <$> f (fromMaybe (guess x) (getter x))
 
 lensWithDefault' ∷ α → (Maybe α → σ → σ) → (σ → Maybe α) → Lens' σ α
-lensWithDefault' = lensWithDefault ∘ const
+lensWithDefault' x = lensWithDefault (const x)
 
 (!?) ∷ [α] → Int → Maybe α
 (!?) xs i =

src/WithPlus.hs

@@ -9,7 +9,7 @@ module WithPlus
     , parseString
     ) where
 
-import BasePrelude hiding (toList, fromList)
+import BasePrelude hiding (toList, fromList, singleton)
 import Prelude.Unicode
 import Data.Monoid.Unicode ((∅))
 import Util (HumanReadable, split)