Use Builder internally for representing Text

This is more efficient since the only thing we ever do is concatenate text,
which `Builder` is optimized for.

src/Dhall.hs

@@ -325,6 +325,7 @@ import Prelude hiding (maybe)
 import qualified Control.Exception
 import qualified Data.Map
 import qualified Data.Text.Lazy
+import qualified Data.Text.Lazy.Builder
 import qualified Data.Vector
 import qualified Dhall.Core
 import qualified Dhall.Import
@@ -448,7 +449,7 @@ double = Type {..}
 text :: Type Text
 text = Type {..}
   where
-    extract (TextLit t) = pure t
+    extract (TextLit t) = pure (Data.Text.Lazy.Builder.toLazyText t)
     extract  _          = empty
 
     expected = Text

src/Dhall/Core.hs

@@ -225,7 +225,7 @@ data Expr a
     -- | > Text                                     ~  Text
     | Text
     -- | > TextLit t                                ~  t
-    | TextLit Text
+    | TextLit Builder
     -- | > TextAppend x y                           ~  x <> y
     | TextAppend (Expr a) (Expr a)
     -- | > TextConcat                               ~  Text/concat
@@ -1954,7 +1954,7 @@ normalize e = case e of
             App TextConcat (ListLit _ xs)
                 | Data.Vector.all isText xs ->
                     case traverse extract xs of
-                        Just ys -> normalize (TextLit (Text.concat (toList ys)))
+                        Just ys -> normalize (TextLit (mconcat (toList ys)))
                         _       -> error "normalize: Malformed `Text/concat`"
               where
                 isText (TextLit _) = True

src/Dhall/Lexer.x

@@ -22,6 +22,7 @@ import Data.ByteString.Lazy (ByteString)
 import Data.Monoid ((<>))
 import Data.Text.Buildable (Buildable(..))
 import Data.Text.Lazy (Text)
+import Data.Text.Lazy.Builder (Builder)
 import Filesystem.Path (FilePath)
 import Numeric.Natural (Natural)
 import Prelude hiding (FilePath)
@@ -33,6 +34,7 @@ import qualified Data.Text
 import qualified Data.Text.Buildable
 import qualified Data.Text.Lazy
 import qualified Data.Text.Lazy.Encoding
+import qualified Data.Text.Lazy.Builder
 import qualified Filesystem.Path.CurrentOS
 import qualified NeatInterpolation
 }
@@ -165,8 +167,11 @@ toText :: ByteString -> Text
 toText = Data.Text.Lazy.Encoding.decodeUtf8
 
 -- TODO: Properly handle errors here
-str :: ByteString -> Text
-str = read . Data.Text.Lazy.unpack . Data.Text.Lazy.Encoding.decodeUtf8
+str :: ByteString -> Builder
+str = Data.Text.Lazy.Builder.fromLazyText
+    . read
+    . Data.Text.Lazy.unpack
+    . Data.Text.Lazy.Encoding.decodeUtf8
 
 -- | `Alex` action for reading the next token
 lexer :: (Token -> Alex a) -> Alex a
@@ -275,7 +280,7 @@ data Token
     | ListConcat
     | Maybe
     | MaybeFold
-    | TextLit Text
+    | TextLit Builder
     | TextConcat
     | Absurd
     | Label Text