Switch from pretty-simple to pretty-show for native output.

Update tests.

Reason:  it turns out that the native output generated by
pretty-simple isn't always readable by the native reader.
According to https://github.com/cdepillabout/pretty-simple/issues/99
it is not a design goal of the library that the rendered values
be readable using 'read'.  This makes it unsuitable for our
purposes.

pretty-show is a bit slower and it uses 4-space indents
(non-configurable), but it doesn't have this serious drawback.

test/command/shift-heading-level-by.md

@@ -9,17 +9,19 @@ title: My title
 ## Second
 ^D
 Pandoc
-  ( Meta
-    { unMeta = fromList
-      [ ( "title", MetaInlines [ Str "My", Space, Str "title" ] ) ]
+  Meta
+    { unMeta =
+        fromList
+          [ ( "title"
+            , MetaInlines [ Str "My" , Space , Str "title" ]
+            )
+          ]
     }
-  )
-  [ Header 2
-    ( "first-heading", [], [] )
-    [ Str "First", Space, Str "heading" ]
-  , Header 3
-    ( "second", [], [] )
-    [ Str "Second" ]
+  [ Header
+      2
+      ( "first-heading" , [] , [] )
+      [ Str "First" , Space , Str "heading" ]
+  , Header 3 ( "second" , [] , [] ) [ Str "Second" ]
   ]
 ```
 
@@ -36,16 +38,22 @@ title: Old title
 # Another top-level heading
 ^D
 Pandoc
-  ( Meta
-    { unMeta = fromList
-      [ ( "title", MetaInlines [ Str "First", Space, Str "heading" ] ) ]
+  Meta
+    { unMeta =
+        fromList
+          [ ( "title"
+            , MetaInlines [ Str "First" , Space , Str "heading" ]
+            )
+          ]
     }
-  )
-  [ Header 1
-    ( "second", [], [] )
-    [ Str "Second" ]
+  [ Header 1 ( "second" , [] , [] ) [ Str "Second" ]
   , Para
-    [ Str "Another", Space, Str "top-level", Space, Str "heading" ]
+      [ Str "Another"
+      , Space
+      , Str "top-level"
+      , Space
+      , Str "heading"
+      ]
   ]
 ```