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/5540.md

@@ -5,10 +5,10 @@ Stay pure!
 \end{lstlisting}
 ^D
 [ CodeBlock
-  ( ""
-  , [ "myfunnylanguage" ]
-  ,
-    [ ( "language", "myfunnylanguage" ) ]
-  ) "Stay pure!"
+    ( ""
+    , [ "myfunnylanguage" ]
+    , [ ( "language" , "myfunnylanguage" ) ]
+    )
+    "Stay pure!"
 ]
 ```