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/4845.md

@@ -3,14 +3,19 @@
 x<a href="/foo"> leading trailing space </a>x
 ^D
 [ Plain
-  [ Str "x"
-  , Space
-  , Link
-    ( "", [], [] )
-    [ Str "leading", Space, Str "trailing", Space, Str "space" ]
-    ( "/foo", "" )
-  , Space
-  , Str "x"
-  ]
+    [ Str "x"
+    , Space
+    , Link
+        ( "" , [] , [] )
+        [ Str "leading"
+        , Space
+        , Str "trailing"
+        , Space
+        , Str "space"
+        ]
+        ( "/foo" , "" )
+    , Space
+    , Str "x"
+    ]
 ]
 ```