day6: add haskell solution

README.md

@@ -11,3 +11,4 @@ https://adventofcode.com/2020/
 | 3 |  `**`  |  `**`   |      |
 | 4 |  `**`  |  `**`   |      |
 | 5 |        |  `**`   |      |
+| 6 |        |  `**`   |      |

day6/day6.hs

@@ -0,0 +1,9 @@
+import AoC
+
+import qualified Data.Set as S
+import           Data.Set (Set)
+
+main = runAoC ((fmap . fmap) S.fromList . splitOnEmptyLines) part1 part2
+    where part1 = countSetsFolded S.union
+          part2 = countSetsFolded S.intersection
+          countSetsFolded f = sum . map (S.size . foldr1 f)