Add "SimpleT" t output form

This new output form allows easy piping to the decode function of the
tricu executable. Includes a new test for roundtrip evaluation of map,
compilation to tree calculus terms, and decoding back to a human
readable string.

src/Eval.hs

@@ -3,8 +3,7 @@ module Eval where
 import Parser
 import Research
 
-import Data.Map (Map)
-import Data.List (foldl')
+import Data.Map  (Map)
 
 import qualified Data.Map as Map
 import qualified Data.Set as Set
@@ -102,11 +101,6 @@ freeVars (SLambda vs b) = foldr Set.delete (freeVars b) vs
 isFree :: String -> TricuAST -> Bool
 isFree x = Set.member x . freeVars
 
-toAST :: T -> TricuAST
-toAST Leaf = TLeaf
-toAST (Stem a) = TStem (toAST a)
-toAST (Fork a b) = TFork (toAST a) (toAST b)
-
 -- We need the SKI operators in an unevaluated TricuAST tree form so that we
 -- can keep the evaluation functions straightforward
 tI :: TricuAST