Add happy 1.20.0

patches/happy-1.20.0.patch

+diff --git a/src/Grammar.lhs b/src/Grammar.lhs
+index d3ce625..5d88511 100644
+--- a/src/Grammar.lhs
++++ b/src/Grammar.lhs
+@@ -30,6 +30,7 @@ Here is our mid-section datatype
+ > import Data.List
+ > import Data.Maybe (fromMaybe)
+ 
++> import Control.Monad
+ > import Control.Monad.Writer
+ 
+ > type Name = Int