Add line numbers to error messages (#919)

Fixes https://github.com/dhall-lang/dhall-haskell/issues/905

For example, this file:

```haskell
[ 1
, 2
, 3
, Bool

+ 1
]
```

... now produces this error message:

```
Use "dhall --explain" for detailed errors

↳ ./example.dhall

Error: ❰+❱ only works on ❰Natural❱s

4:   Bool
5:
6:
7:
8:
9: + 1

/Users/gabriel/proj/dhall-haskell/dhall/example.dhall:4:3
```

dhall/src/Dhall/Parser/Combinators.hs

@@ -38,6 +38,7 @@ import qualified Text.Megaparsec.Char
 import qualified Text.Parser.Char
 import qualified Text.Parser.Combinators
 import qualified Text.Parser.Token.Style
+import qualified Text.Printf
 
 -- | Source code extract
 data Src = Src !Text.Megaparsec.SourcePos !Text.Megaparsec.SourcePos Text
@@ -68,7 +69,7 @@ laxSrcEq (Src p q _) (Src p' q' _) = eq p  p' && eq q q'
 
 instance Pretty Src where
     pretty (Src begin _ text) =
-            pretty (Dhall.Util.snip (prefix <> text))
+            pretty (Dhall.Util.snip numberedLines)
         <>  "\n"
         <>  pretty (Text.Megaparsec.sourcePosPretty begin)
       where
@@ -76,6 +77,28 @@ instance Pretty Src where
           where
             n = Text.Megaparsec.unPos (Text.Megaparsec.sourceColumn begin)
 
+        ls = Data.Text.lines (prefix <> text)
+
+        numberOfLines = length ls
+
+        minimumNumber =
+            Text.Megaparsec.unPos (Text.Megaparsec.sourceLine begin)
+
+        maximumNumber = minimumNumber + numberOfLines - 1
+
+        numberWidth :: Int
+        numberWidth =
+            truncate (logBase (10 :: Double) (fromIntegral maximumNumber)) + 1
+
+        adapt n line = Data.Text.pack outputString
+          where
+            inputString = Data.Text.unpack line
+
+            outputString =
+                Text.Printf.printf ("%" <> show numberWidth <> "d: %s") n inputString
+
+        numberedLines = Data.Text.unlines (zipWith adapt [minimumNumber..] ls)
+
 {-| A `Parser` that is almost identical to
     @"Text.Megaparsec".`Text.Megaparsec.Parsec`@ except treating Haskell-style
     comments as whitespace

dhall/src/Dhall/Tutorial.hs

@@ -314,7 +314,7 @@ import Dhall
 -- - Bool
 -- + Natural
 -- ...
--- 1 : Bool
+-- 1: 1 : Bool
 -- ...
 -- (input):1:1
 -- ...
@@ -514,7 +514,8 @@ import Dhall
 -- *** Exception:
 -- ...Error...: An empty list requires a type annotation
 -- ...
--- []
+-- 1: []
+-- ...
 -- (input):1:1
 --
 -- Also, list elements must all have the same type.  You will get an error if
@@ -527,7 +528,7 @@ import Dhall
 -- - Natural
 -- + Bool
 -- ...
---     True
+-- 1:     True
 -- ...
 -- (input):1:5
 -- ...